Stabilize sse4a and tbm target features

[sayantn]

This PR stabilizes the feature flag sse4a_target_feature and tbm_target_feature (tracking issue #44839).

Public API

The 2 x86 target features sse4a and tbm

Also, these were added in LLVM2.6 and LLVM3.4-rc1, respectively, and as the minimum LLVM required for rustc is LLVM19, we are safe in that front too!

As all of the required tasks have been done (adding the target features to rustc, implementing their runtime detection in std_detect and implementing the associated intrinsics in core_arch), these target features can be stabilized now. The intrinsics were stabilized long ago, in 1.27.0

cc @rust-lang/lang

@rustbot label I-lang-nominated
r? lang

[rustbot]

r? @petrochenkov

rustbot has assigned @petrochenkov.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rustbot]

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

cc @Amanieu, @folkertdev, @sayantn

[traviscross]

Thanks. This needs a PR to the Reference.

cc @tgross35 @RalfJung @Amanieu @workingjubilee @rust-lang/lang-docs

[RalfJung]

The usual question for target feature stabilizations:
Are there any ABI concerns? Is there any potential unsoundness or other problems when mixing code built with and without these features?