Remove duplication of RTCD configs in the cmake build.

Consolidate the CPU and RTCD related build configuration code
in cpu.cmake and rtcd_config.cmake instead of having files for
each target CPU.

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

Change-Id: I8aad401f9fca60ba735360ca4947cd1c41c4fa63
18 files changed