Rollup merge of #145656 - folkertdev:stabilize-s390x-vector, r=Amanieu

Stabilize s390x `vector` target feature and `is_s390x_feature_detected!` macro

closes rust-lang/rust#145649
closes rust-lang/rust#135413
cc: rust-lang/rust#130869
reference PR: rust-lang/reference#1972

# Stabilization report

## Summary

This PR stabilizes the following s390x target features:

- `vector`
- `vector-enhancements-1`
- `vector-enhancements-2`
- `vector-enhancements-3`
- `vector-packed-decimal`
- `vector-packed-decimal-enhancement`
- `vector-packed-decimal-enhancement-2`
- `vector-packed-decimal-enhancement-3`
- `nnp-assist`
- `miscellaneous-extensions-2`
- `miscellaneous-extensions-3`
- `miscellaneous-extensions-4`

Additionally, it stabilizes the `std::arch::is_s390x_feature_detected!` macro itself and stably accepts the target features listed above.

## Tests & ABI details

Only the `vector` target feature changes the ABI, much like e.g. `avx2` it will, depending on the ABI, pass vector types in vector registers. This behavior is tested extensively:

- [tests/assembly-llvm/s390x-vector-abi.rs](https://github.com/rust-lang/rust/blob/22a86f8280becb12c34ee3efd952baf5cf086fa0/tests/assembly-llvm/s390x-vector-abi.rs)
- [tests/codegen-llvm/s390x-simd.rs](https://github.com/rust-lang/rust/blob/22a86f8280becb12c34ee3efd952baf5cf086fa0/tests/assembly-llvm/s390x-vector-abi.rs)
- [tests/ui/abi/simd-abi-checks-s390x.rs ](https://github.com/rust-lang/rust/blob/22a86f8280becb12c34ee3efd952baf5cf086fa0/tests/ui/abi/simd-abi-checks-s390x.rs )

The remaining features don't influence the ABI, they only influence instruction selection. In stdarch we test that the expected instructions are in fact generated when the target feature is enabled.

## Implementation history

For `is_s390x_feature_detected!`:

- #1699
- rust-lang/rust#138275
- #1720
- #1832

For `vector` and friends

- rust-lang/rust#127506
- rust-lang/rust#135630
- rust-lang/rust#141250

## Unresolved questions

There is a fixme in [tests/ui/abi/simd-abi-checks-s390x.rs](https://github.com/rust-lang/rust/blob/22a86f8280becb12c34ee3efd952baf5cf086fa0/tests/ui/abi/simd-abi-checks-s390x.rs):

```
// FIXME: +soft-float itself doesn't set -vector
//`@[z13_soft_float]` compile-flags: --target s390x-unknown-linux-gnu -C target-cpu=z13 -C target-feature=-vector,+soft-float
//`@[z13_soft_float]` needs-llvm-components: systemz
```

I'm not sure whether that blocks stabilization?

---

The implementation first extracts the listed target features into their own `s390x_target_feature_vector` rust feature, and then stabilizes that. best reviewed commit-by-commit

r? `@Amanieu`
cc `@uweigand`  `@taiki-e`

Author: matthiaskrgr

crates/core_arch/src/lib.rs

@@ -22,7 +22,6 @@
     arm_target_feature,
     mips_target_feature,
     powerpc_target_feature,
-    s390x_target_feature,
     loongarch_target_feature,
     wasm_target_feature,
     abi_unadjusted,
@@ -64,11 +63,7 @@
 )]
 #![cfg_attr(
     test,
-    feature(
-        stdarch_arm_feature_detection,
-        stdarch_powerpc_feature_detection,
-        stdarch_s390x_feature_detection
-    )
+    feature(stdarch_arm_feature_detection, stdarch_powerpc_feature_detection,)
 )]
 
 #[cfg(test)]