Require explicit request for generic CPU at cmake generation time.

x86 and x86_64 support is now working for most platforms. Remove the
automatic fallback to a generic unoptimized build, and require users
to explicitly request generic to build a generic library and tools.

BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=76

Change-Id: I7b36394dc28aa9b729d2b2504146d02f017c9a0b
1 file changed