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
2 files changed