Fix PIC relocations for non-Bsymbolic builds

The ARM assembly contains a couple of hand-coded PC relative references
to data defined in other object files. Make sure the externs are tagged
as hidden so the linker knows that it can resolve the reference locally
at build time.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@gmail.com>
Bug: 16853291
Change-Id: If7bc8e5bb6a2c63b9df33666d56e6c42f398fd47