[ARM] Do not test for CPUs, use SubtargetFeatures (Part 1). NFCI

This is a cleanup commit similar to r271555, but for ARM.

The end goal is to get rid of the isSwift / isCortexXY / isWhatever methods.

Since the ARM backend seems to have quite a lot of calls to these methods, I
intend to submit 5-6 subtarget features at a time, instead of one big lump.

Differential Revision: http://reviews.llvm.org/D21432

llvm-svn: 273544