Use improved __sync_*() intrinsics for MIPS in userspace as well.

r251524 introduced custom tailored versions for MIPS of these functions
for kernel-space code. We can just reuse them in userspace as well.
  Add new system call - aio_mlock(). The name speaks for itself. It allows
to perform the mlock(2) operation, which can consume a lot of time, under
control of aio(4).

Reviewed by:	kib, jilles
Sponsored by:	Nginx, Inc.
aio_mlock() added:
  - Regen for r251526.
  - Bump __FreeBSD_version.
sigaction(2): Document various non-POSIX functions as async-signal safe.