AndroidKernel.mk: Allow make headers for different arch

Allow headers (sanitized kernel header generation for
userspace consumption) for a different architecture than
the one use for the kernel build.

This is normally not recommended, but is useful for bring-up
purposes.

Change-Id: Ia2cfffb2e8e9c255cb1567664e88f7df0e8bb126
Signed-off-by: David Ng <dave@codeaurora.org>