Provide generic sub-word atomic *cmpset

Provide *cmpset_{8,16} as wrappers around atomic_fcmpset_32. Initial users
will be mips and sparc64, and perhaps parts of powerpc.

This are not for general consumption; machine/atomic.h should include this
header as needed to provide atomic_{,f}cmpset_{8,16} and machine/atomic.h
should provide acq_ and rel_ variants.

Reviewed by: jhibbits, kib
Differential Revision: https://reviews.freebsd.org/D21822