Added new atomic functions, renamed some old ones. Some #defines have
been added for backward compatibility.
Merged the pre- and post-ARMv6 implementations into a single file.
Renamed the semi-private __android_membar_full_smp to USE_SCREAMING_CAPS
since that's more appropriate for a macro.
Added lots of comments.
Note Mac OS X primitives have not been tested.
Change-Id: If827260750aeb61ad5c2b760e30658e29dbb26f2