[ARM] Add knowledge of FPU subtarget features to TargetParser

Add getFPUFeatures to TargetParser, which gets the list of subtarget features
that are enabled/disabled for each FPU, and use it when handling the .fpu
directive.

No functional change in this commit, though clang will start behaving
differently once it starts using this.

Differential Revision: http://reviews.llvm.org/D10237

llvm-svn: 239150