Atomic/SMP update, part 2.

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