Stabilize x86/x86_64 SIMD

This commit stabilizes the SIMD in Rust for the x86/x86_64 platforms. Notably
this commit is stabilizing:

* The `std::arch::{x86, x86_64}` modules and the intrinsics contained inside.
* The `is_x86_feature_detected!` macro in the standard library
* The `#[target_feature(enable = "...")]` attribute
* The `#[cfg(target_feature = "...")]` matcher

Stabilization of the module and intrinsics were primarily done in
rust-lang-nursery/stdsimd#414 and the two attribute stabilizations are done in
this commit. The standard library is also tweaked a bit with the new way that
stdsimd is integrated.

Note that other architectures like `std::arch::arm` are not stabilized as part
of this commit, they will likely stabilize in the future after they've been
implemented and fleshed out. Similarly the `std::simd` module is also not being
stabilized in this commit, only `std::arch`. Finally, nothing related to `__m64`
is stabilized in this commit either (MMX), only SSE and up types and intrinsics
are stabilized.

Closes #29717
Closes #44839
Closes #48556

src/libstd/lib.rs

@@ -525,7 +525,7 @@ mod coresimd {
 #[unstable(feature = "stdsimd", issue = "48556")]
 #[cfg(all(not(stage0), not(test)))]
 pub use stdsimd::simd;
-#[unstable(feature = "stdsimd", issue = "48556")]
+#[stable(feature = "simd_arch", since = "1.27.0")]
 #[cfg(all(not(stage0), not(test)))]
 pub use stdsimd::arch;