Check that a working android toolchain was found

If the user selects a nonexistent ARCH, previously the call that
sets TOOLCHAINPREFIX would set it to contain a long error message,
which makes it quite hard to figure out what actually went wrong.

Instead check that CXX=$(TOLCHAINPREFIX)g++ actually points to a
working executable, and abort immediately if this isn't true.

Review at https://rbcommons.com/s/OpenH264/r/698/.