Revert: Resolve fence TODO.
Revert: Resolve fence TODO.

Linux has had a fast fence membarrier operation for rseq since
https://github.com/torvalds/linux/commit/2a36ab717e8fe678d98f81c14a0b124712719840
(5.10).

PiperOrigin-RevId: 454174741
Change-Id: Ia376836e93611c2a49b86d3adaeda56bece2a4e1

google/tcmalloc,google/tcmalloc,google/tcmalloc