[ARM] Use DWARF register numbers for CFI directives in ELF assembly

It seems gas can't handle CFI directives with VFP register names ("d12", etc.).
This broke us trying to build Chromium for Android after 201423.

A gas bug has been filed: https://sourceware.org/bugzilla/show_bug.cgi?id=16694

compnerd suggested making this conditional on whether we're using the integrated
assembler or not. I'll look into that in a follow-up patch.

Differential Revision: http://llvm-reviews.chandlerc.com/D3049