Add basic armv7 linux support to the cmake build.

Requires new cmake toolchain file  armv7-linux-gcc.cmake:
$ cmake path/to/aom -DCMAKE_TOOLCHAIN_FILE=path/to/aom/build/cmake/toolchains/armv7-linux-gcc.cmake

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

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

Change-Id: Id2d0022fd5b4615fef7ea0ef38980cbdfd957c0e
2 files changed