[TargetParser] Split AArch64TargetParser from ARMTargetParser

AArch64TargetParser reuses data structures and some data from ARMTargetParser,
which causes more problems than it solves. This change separates them.

Code which is common to ARM and AArch64 is moved to ARMTargetParserCommon
which both ARMTargetParser and AArch64TargetParser use.

Some of the information in AArch64TargetParser.def was unused or nonsensical
(CPU_ATTR, ARCH_ATTR, ARCH_FPU) because it reused data strutures from
ARMTargetParser where some of these make sense. These are removed.

Differential Revision: https://reviews.llvm.org/D137924