Slight thread.h tweaks
* Replace an asm barrier with a more portable gcc intrinsic.
  Timings appear not to suffer.
* Replace Windows _ReadWriteBarrier with MemoryBarrier; reading docs indicate
  that RWB only affects compiler semantics, not processor semantics.
New build flag OIIO_THREAD_ALLOW_DCLP, if set to 0, will modify spin_mutex
and spin_rw_mutex to avoid the double checked locking pattern, which we
think is correct, but throws off Thread Sanitizer and possibly could be
a problem for other tools in the future.