We'll need the fallbacks in any case. However, once we've
decided on how to specify the CPU features of the user
machine we can use the nicer LZCNT/TZCNT/POPCNT as well.

Adds cmov, bsf, and bsr instructions.

Calls a popcount helper function for machines without SSE4.2.

Not handling bswap yet (which can also take i16 params).

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3882
R=stichnot@chromium.org, wala@chromium.org

Review URL: https://codereview.chromium.org/390443005