SHARED is only supported when the pthread library does support spinlock,
while in all other case it falls back into using atomic locks. Providing
support only for a small fraction of environments without reporting errors
or providing any alternative on other systems is difficult to accept.
I think
WHAT: Add a new "opal/threads/spinlock.h" header to OPAL that will typically
use the OS spinlock primitives if present.
WHY: opal_mutex_t is too slow for some use cases and opal_atomic_lock_t is
inefficiently implemented for most architectures
WHEN: timeout is after next week's engineering call
