Add mips64 support to the cmake build.

Requires use of new cmake toolchain file:
$ cmake path/to/aom -DCMAKE_TOOLCHAIN_FILE=path/to/aom/build/cmake/toolchains/mips64-linux-gcc.cmake

MSA is supported via addition of -DENABLE_MSA=1. Support for i6400
and p6600 targets can be enabled by passing -DMIPS_CPU=i6400 and
-DMIPS_CPU=p6600 respectively.

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

Change-Id: I7b8949a07e9be63cd6177e4ff4dbc02180f12a60
4 files changed