stabilize s390x vector registers

[folkertdev]

tracking issue: #133416

Stabilizes s390x vector registers, e.g.

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 vregs.

Implementation history

• Support input/output in vector registers of s390x inline assembly (under asm_experimental_reg feature) #131664
• Add f16 inline ASM support for s390x #150826

cc @uweigand @taiki-e
r? @Amanieu