Move aom_dsp cmake stuff into aom_dsp/aom_dsp.cmake.

- Move source list vars.
- Split source list vars into common/decoder/encoder sources.
- Move target definitions into function.
- Split targets into common/decoder/encoder targets.
- Update CMakeLists.txt to include aom_dsp.cmake and call
  setup_aom_dsp_targets() at the appropriate time.

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

Change-Id: I434d2e76b04cbccdbdcec7ca818b36b8ee1f840e
3 files changed