Expand cmake assembler support.
- Stop acting as if Yasm is the only assembler. - Kill generation and report error when yasm is not found for x86 and x86_64 (remove the generic fallback). - Use $AOM_AS_FLAGS to pass assembler specific flags. - Add include guard in aom_optimization.cmake. BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=76 Change-Id: Ic68d6c81071c24a8ceb6806d04ab8959be97d876