Allow control of cross compile prefix in cmake arm linux builds.

Via the CROSS CMake variable. Supported in the armv7 and arm64
cmake toolchain files.

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

Change-Id: I37587aeaf6e773c5f7b3f5fa67277ddc44c9195a
2 files changed