* libc/include/pthread.h: Add pthread barriers,
rwlocks, and spinlocks. Change const to
_CONST and prefix parameter names with
an underscore.
* libc/include/sys/types.h: Add id and attribute
types for barriers, wrlocks, and spinlocks.
* libc/include/sys/features.h: Enable barriers,
wrlocks, and spinlocks for RTEMS.