Allow arm prebuilts for x86_arm builds

But only for static and shared libraries -- we disable building other
types of modules for arm, so don't support the prebuilts for them
either.

This is needed so that we can build these libraries under Soong, and
expose them to Make using BUILD_PREBUILT.

Bug: 27526885
Change-Id: I4f6ec922aed37591b6257b7022b81fb65d05b0fe