LLVM's default bad-alloc handler may throw if exceptions are enabled,
and `operator new` isn't hooked at all by default. Now we register our
own handler that prints a message similar to fatal errors, then aborts.
We also call the function that registers the C++ `std::new_handler`.

The bad-alloc installer was incorrectly asserting that the other handler
isn't set yet, instead of checking its own, but we can avoid that by
changing the order we install them.

Ref: llvm/llvm-project#83040

Add as_(mut_)ptr and as_(mut_)slice to raw array pointers

Hey, first time contributing to the standard libraries so not completely sure about the process.

These functions are complementary to the ones being added in rust-lang#74265 . I found them missing on array pointers.

See also:
- ACP: rust-lang/libs-team#321
- Tracking issue: rust-lang#119834

…ulacrum

Respect stage0 sysroot when compiling rmake.rs with COMPILETEST_FORCE_STAGE0

Context: <https://rust-lang.zulipchat.com/#narrow/stream/326414-t-infra.2Fbootstrap/topic/stage0.20compiletest.20broken>.
> cg_clif uses `COMPILETEST_FORCE_STAGE0=1 ./x.py test --stage 0` for running the rustc test suite. With the introduction of rmake.rs this broke. `librun_make_support.rlib` is compiled using the bootstrap rustc wrapper which sets `--sysroot build/aarch64-unknown-linux-gnu/stage0-sysroot`, but then compiletest will compile `rmake.rs` using the sysroot of the bootstrap compiler causing it to not find the `libstd.rlib` against which `librun_make_support.rlib` is compiled.

cc ``@bjorn3``

Fixes rust-lang#122196.

…et, r=Mark-Simulacrum

mir-opt: always run tests for the current target

Currently, `./x.py test tests/mir-opt` runs only the tests for the current target, and `./x.py test tests/mir-opt --bless` runs tests for a representative set of targets. That representative set does not include the current target however, which means `--bless` can succeed when tests fail without it. This PR ensures we run the current target always.

Fixes rust-lang#122292
cc ``@RalfJung``

Register LLVM handlers for bad-alloc / OOM

LLVM's default bad-alloc handler may throw if exceptions are enabled,
and `operator new` isn't hooked at all by default. Now we register our
own handler that prints a message similar to fatal errors, then aborts.
We also call the function that registers the C++ `std::new_handler`.

Fixes rust-lang#121305
Cc llvm/llvm-project#85281
r? ``@nikic``

…gic, r=fmease

Move check-cfg diagnostic logic into a separate file

as well as adding some triagebot mentions (for me) for check-cfg related files.

``@rustbot`` label +F-check-cfg