Add a mutex and atomic counter abstraction and implementations

Add a mutex and atomic counter abstraction and implementations

These abstractions can be used to implement an efficient resource
reference counter and simple mutual exclusion.  On pthreads we use
pthread_mutex_t, except when we are also on glibc and can directly
use its asm/atomic.h definitions.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>