[ARM][AArch64] generate subtarget feature flags

This patch aims to reduce a lot of the boilerplate around adding new subtarget
features. From the SubtargetFeatures tablegen definitions, a series of calls to
the macro GET_SUBTARGETINFO_MACRO are generated in
ARM/AArch64GenSubtargetInfo.inc.  ARMSubtarget/AArch64Subtarget can then use
this macro to define bool members and the corresponding getter methods.

Some naming inconsistencies have been fixed to allow this, and one unused
member removed.

This implementation only applies to boolean members; in future both BitVector
and enum members could also be generated.

Differential Revision: https://reviews.llvm.org/D120906