[ARM] Correct ARMv8*-A optional extension definitions in TargetParser

The ARMv8*-A descriptions in the ARM and AArch64 TargetParsers are incorrect
architecturally and mismatched to the backend descriptions.

RAS is an optional extension to ARMv8-A and ARMv8.1-A and mandatory in
ARMv8.2-A. Correct the ARMTargetParser descriptions which had this as enabled
by default in the earlier versions.

The FP16 and SPE extensions are optional in ARMv8.2-A and the backend defaults
them as off. They are not available as extensions to earlier ARMv8-A versions.
Correct the AArch64TargetParser which had these as enabled by default in all
ARMv8-A definitions.

These macros are only used to define preprocessor macros. There are no macros
yet as ACLE has not caught up with ARMv8.2-A so not possible to add a test.

Differential Revision: https://reviews.llvm.org/D23500

llvm-svn: 279078