util: move fuchsia futex implementation

Also fixes futex.h.

Bug:460783239
Change-Id: Ia834b5a9eb6145c543db883f624beea89b21563e
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1429547
Reviewed-by: David Gilhooley <dgilhooley@google.com>
3 files changed