[libc][cpp] add `atomic_signal_fence` (#82138)

Add `atomic_signal_fence`. This will be useful in
https://gustedt.gitlabpages.inria.fr/c23-library/#memset_explicit.

GitOrigin-RevId: 8f7ae64ea108de54d9aad963c55e1aef7dc62b86
1 file changed