[ARM] Use symbolic register names in .cfi directives only with IAS (PR19110)

This is a follow-up to r203635. Saleem pointed out that since symbolic register
names are much easier to read, it would be good if we could turn them off only
when we really need to because we're using an external assembler.

Differential Revision: http://llvm-reviews.chandlerc.com/D3056