[ARM] Use TargetParser to determine FPU subtarget features

The main effect of this is to fix anomalies where certain -mfpu options didn't
disable everything that they should causing strange behaviour when combined
with -mcpu or -march values that themselves enabled fpu subtarget features,
e.g. -mfpu=fpv5-dp-d16 with -march=armv7em previously behaved the same as
-mfpu=fpv5-sp-d16 due to fp-only-sp not being disabled.

Invalid -mfpu options now also give an error, which is consistent with the
handling of the .fpu directive.

Differential Revision: http://reviews.llvm.org/D10239

llvm-svn: 239152