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.
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.

scott-wilson/oiio,mcanthony/oiio,micler/oiio,YangYangTL/oiio,mcanthony/oiio,micler/oiio,scott-wilson/oiio,YangYangTL/oiio,lgritz/oiio,bdeluca/oiio,scott-wilson/oiio,OpenImageIO/oiio,sambler/oiio,OpenImageIO/oiio,OpenImageIO/oiio,micler/oiio,scott-wilson/oiio,micler/oiio,mcanthony/oiio,lgritz/oiio,YangYangTL/oiio,sambler/oiio,cwilling/oiio,lgritz/oiio,cwilling/oiio,mcanthony/oiio,bdeluca/oiio,sambler/oiio,YangYangTL/oiio,OpenImageIO/oiio,lgritz/oiio,cwilling/oiio,bdeluca/oiio,bdeluca/oiio,cwilling/oiio,sambler/oiio