Using __builtin_clz on gcc/clang, and a fallback implementation
otherwise.
Also redefine WebRtcSpl_GetSizeInBits(x) as simply 32 - clz32(x).
BUG=chromium:601787
Review-Url: https://codereview.webrtc.org/2014023002
Cr-Commit-Position: refs/heads/master@{#13014}