stabilize s390x specific target-feature backchain

[fneddy]

This makes backchain stable for -Ctarget-feature=+backchain and
#[target_feature(enable = "backchain")].

What changes:

• Mark backchain as stable in the s390x target feature list.
• Update the packed-stack compatibility check to read backchain from
  sess.target_features.
• Remove UI expectations for the old unstable-feature warning.

Tests:

• tests/ui/target-feature/packedstack-combinations.rs
• tests/codegen-llvm/backchain.rs
• tests/assembly-llvm/s390x-backchain-toggle.rs

Tracking issue: #158014

Note:
As this is an architecture specific option I did NOT update the the docs and
did NOT write a stabilization report

[rustbot]

r? @petrochenkov

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[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

fmt: checked 6968 files
tidy check
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy: Skipping binary file check, read-only filesystem
tidy [style (tests)]: /checkout/tests/ui/target-feature/packedstack-combinations.rs:2: line longer than 100 chars
tidy [style (tests)]: FAIL
removing old virtual environment
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'venv'
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'virtualenv'
Requirement already satisfied: pip in ./build/venv/lib/python3.10/site-packages (26.1.1)
Collecting pip
---
info: ✓ ES-Check passed! All files are ES10 compatible.
typechecking javascript files
tidy: The following check failed: style (tests)
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy --root-path=/checkout --cargo-path=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo --output-dir=/checkout/obj/build --concurrency=4 --npm-path=/node/bin/yarn --ci=true --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1625:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1646:29

--- BACKTRACE vvv
   0: <bootstrap::utils::exec::DeferredCommand>::finish_process
             at /checkout/src/bootstrap/src/utils/exec.rs:939:17
   1: <bootstrap::utils::exec::DeferredCommand>::wait_for_output::<&bootstrap::utils::exec::ExecutionContext>
             at /checkout/src/bootstrap/src/utils/exec.rs:831:21
   2: <bootstrap::utils::exec::ExecutionContext>::run
             at /checkout/src/bootstrap/src/utils/exec.rs:741:45
   3: <bootstrap::utils::exec::BootstrapCommand>::run::<&bootstrap::core::builder::Builder>
             at /checkout/src/bootstrap/src/utils/exec.rs:339:27
   4: <bootstrap::core::build_steps::test::Tidy as bootstrap::core::builder::Step>::run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:1646:29
   5: <bootstrap::core::builder::Builder>::ensure::<bootstrap::core::build_steps::test::Tidy>
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1597:36
   6: <bootstrap::core::build_steps::test::Tidy as bootstrap::core::builder::Step>::make_run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:1568:21
   7: <bootstrap::core::builder::StepDescription>::maybe_run
             at /checkout/src/bootstrap/src/core/builder/mod.rs:476:13
   8: bootstrap::core::builder::cli_paths::match_paths_to_steps_and_run
             at /checkout/src/bootstrap/src/core/builder/cli_paths.rs:232:18
   9: <bootstrap::core::builder::Builder>::run_step_descriptions
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1140:9
  10: <bootstrap::core::builder::Builder>::execute_cli
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1119:14
  11: <bootstrap::Build>::build
             at /checkout/src/bootstrap/src/lib.rs:803:25
  12: bootstrap::main
             at /checkout/src/bootstrap/src/bin/main.rs:130:11
  13: <fn() as core::ops::function::FnOnce<()>>::call_once
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/core/src/ops/function.rs:250:5
  14: std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/sys/backtrace.rs:166:18
  15: std::rt::lang_start::<()>::{closure#0}
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/rt.rs:206:18
  16: <&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe as core::ops::function::FnOnce<()>>::call_once
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/core/src/ops/function.rs:287:21
  17: std::panicking::catch_unwind::do_call::<&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe, i32>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panicking.rs:581:40
  18: std::panicking::catch_unwind::<i32, &dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panicking.rs:544:19
  19: std::panic::catch_unwind::<&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe, i32>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panic.rs:359:14
  20: std::rt::lang_start_internal::{closure#0}
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/rt.rs:175:24
  21: std::panicking::catch_unwind::do_call::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panicking.rs:581:40
---
  28: __libc_start_main
  29: _start


Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:02:37
  local time: Tue Jun 30 12:05:56 UTC 2026
  network time: Tue, 30 Jun 2026 12:05:56 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

[ehuss]

As this is an architecture specific option I did NOT update the the docs and
did NOT write a stabilization report

Target features always need to be documented (example from a past s390x stabilization).

Also, since these always need a lang FCP, they generally need a written proposal as to what is being stabilized in order for them to make a decision.