wasm: Mark simd intrinsics as stable

[alexcrichton]

This is a follow-up from rust-lang/rust#74372 which has finished FCP for
the stabilization of wasm intrinsics. This marks them all stable, as-is
and additionally marks the functions which create integer vectors as
const-stable as well. The only remaining unstable bits are that
f32x4 and f64x2 are const-unstable. Mostly just because I couldn't
figure out how to make them const-stable.

[rust-highfive]

r? @Amanieu

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[Amanieu]

What is the problem with const f32x4/f64x2 and what does it have to do with rust-lang/rust#73288?

[alexcrichton]

Attempting to const-stabilize them as-is yields:

error: `simd128::<impl simd::f32x4>::v128` is not yet stable as a const fn
   --> crates/core_arch/src/wasm32/simd128.rs:723:5
    |
723 |     simd::f32x4(a0, a1, a2, a3).v128()
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: Const-stable functions can only call other const-stable functions

Attempting to const-stabilize the v128 helper yields:

error: const-stable function cannot use `#[feature(const_fn_transmute)]`
  --> crates/core_arch/src/wasm32/simd128.rs:59:30
   |
59 |                       unsafe { mem::transmute(self) }
   |                                ^^^^^^^^^^^^^^^^^^^^

Basically since v128 is a i32x4 under the hood, I don't think there's a const-stable way in Rust to convert from a float to an integer using a bitcast at compile time.

I was trying to follow the same reasons as this unstable annotation but it appears I got the issues mixed up. The issue that points to is rust-lang/rust#72447. I'll update the links here to point to that issue.