Rollup merge of #91355 - alexcrichton:stabilize-thread-local-const, r=m-ou-se

Rollup merge of #91355 - alexcrichton:stabilize-thread-local-const, r=m-ou-se

std: Stabilize the `thread_local_const_init` feature

This commit is intended to follow the stabilization disposition of the
FCP that has now finished in #84223. This stabilizes the ability to flag
thread local initializers as `const` expressions which enables the macro
to generate more efficient code for accessing it, notably removing
runtime checks for initialization.

More information can also be found in #84223 as well as the tests where
the feature usage was removed in this PR.

Closes #84223