[PATCH] Clang: Introduce cortex-a15 support

Summary:
This is really just treating cortex-a15 as cortex-a9. We have issues with modules that have LOCAL_CLANG := true set
when building with -mcpu=cortex-a15 because our local clang does not have proper cortex-a15 support as upstream clang has.
This will fix builds, but should not be considered a real fix (why real a15 support is upstream now). The original patch and
discussion may be found here (link to gmane for readability): http://comments.gmane.org/gmane.comp.compilers.clang.scm/57249

Test Plan: Test manta build

Reviewers: randall.rushing, Mustaavalkosta, Roach, rlasalle15, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D246