Add basic arm64 linux support to the cmake build.

Requires new cmake toolchain file arm64-linux-gcc.cmake:
$ cmake path/to/aom
-DCMAKE_TOOLCHAIN_FILE=path/to/aom/build/cmake/toolchains/arm64-linux-gcc.cmake

Above requires aarch64-linux-gnu-gcc toolchain is installed and its
bin dir added to PATH.

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

Change-Id: I6c7e297a67415008583b7ac2cd8aea528787e6d0