Separate LIO_SYNC processing into a separate function aio_process_sync(),
and rename aio_process() into aio_process_rw().

Reviewed by:	kib
Sponsored by:	Nginx, Inc.
  Make sys_mlock() function just a wrapper around vm_mlock() function
that does all the job.

Reviewed by:	kib, jilles
Sponsored by:	Nginx, Inc.
Add proper __sync_*() intrinsics for MIPS.

To make <stdatomic.h> work on MIPS (and ARM) using GCC, we need to
provide implementations of the __sync_*() functions. I already added
these functions for 4 and 8 byte types to libcompiler-rt some time ago,
based on top of <machine/atomic.h>.

Unfortunately, <machine/atomic.h> only provides a subset of the features
needed to implement <stdatomic.h>. This means that in some cases we had
to do compare-and-exchange calls in loops, where a simple ll/sc would
suffice.

Also implement these functions for 1 and 2 byte types. MIPS only
provides ll/sc instructions for 4 and 8 byte types, but this is of
course no limitation. We can simply load 4 bytes and use some bitmask
tricks to modify only the bytes affected.

Discussed on:	mips, arch
Tested with:	QEMU