Create x86_64-asan-linux-gnu target which enables ASAN by default

[jakos-sec]

As suggested, in order to distribute sanitizer instrumented standard libraries without introducing new rustc flags, this adds a new dedicated target. With the target, we can distribute the instrumented standard libraries through a separate rustup component.

[rustbot]

This PR changes how LLVM is built. Consider updating src/bootstrap/download-ci-llvm-stamp.

These commits modify compiler targets.
(See the Target Tier Policy.)

[rustbot]

r? @mati865

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[rust-log-analyzer]

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

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[TIMING:end] tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
Diff in /checkout/compiler/rustc_target/src/spec/targets/x86_64_asan_linux_gnu.rs:1:
 use crate::spec::{
-    Arch, Cc, LinkerFlavor, Lld, SanitizerSet, StackProbeType, Target, TargetOptions, TargetMetadata, base,
+    Arch, Cc, LinkerFlavor, Lld, SanitizerSet, StackProbeType, Target, TargetMetadata,
+    TargetOptions, base,
 };
 
 pub(crate) fn target() -> Target {
Diff in /checkout/src/bootstrap/src/core/build_steps/llvm.rs:1293:
             "x86_64",
             &["asan", "dfsan", "lsan", "msan", "safestack", "tsan", "rtsan"],
         ),
-        "x86_64-asan-linux-gnu" => common_libs(
-            "linux",
-            "x86_64",
-            &["asan"],
-        ),
+        "x86_64-asan-linux-gnu" => common_libs("linux", "x86_64", &["asan"]),
         "x86_64-unknown-linux-musl" => {
             common_libs("linux", "x86_64", &["asan", "lsan", "msan", "tsan"])
         }
fmt: checked 6588 files
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:00:49
  local time: Thu Dec  4 14:23:24 UTC 2025

[mati865]

Thanks.

You will have to modify this PR to meet the guidelines outlined at https://doc.rust-lang.org/rustc/target-tier-policy.html#adding-a-new-target.

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.