Rollup merge of #154184 - folkertdev:stabilize-s390x-vector-registers, r=Amanieu

stabilize s390x vector registers

tracking issue: rust-lang/rust#133416
reference PR: rust-lang/reference#2215

Stabilizes s390x vector registers, e.g.

```rust
unsafe fn vreg_128(x: i128) -> i128 {
    let y;
    asm!("vlr {}, {}", out(vreg) y, in(vreg) x);
    y
}
```

The types that are accepted for vreg registers are

- all float types `f16`,  `f32`, `f64`, `f128`
- integer types `i32`, `i64` and `i128` and their unsigned counterparts
- integer vector types `i8x16`, `i16x8`, `i32x4`, `i64x2` and their unsigned counterparts
- float vector types `f16x8`, `f32x4` and `f64x2`

Support for all of these is tested in https://github.com/rust-lang/rust/blob/main/tests/assembly-llvm/asm/s390x-types.rs, and the types correspond with the LLVM definition in https://github.com/llvm/llvm-project/blob/df9eb79970c012990e829d174d181d575d414efe/llvm/lib/Target/SystemZ/SystemZRegisterInfo.td#L312-L339

The `f16`, `f16x8` and `f128` types are unstable, and so can't be used on stable in practice. They do show up in some error messages though.

`vreg` was previously only accepted as a clobber.

---

Currently the vector types in `core::arch::s390x` are still unstable. Separately stabilizing `vreg` is still useful because scalar types can also be put into `vreg`s.

## Implementation history

- rust-lang/rust#131664
- rust-lang/rust#150826

cc @uweigand @taiki-e
r? @Amanieu

Author: JonathanBrouwer