[ARM] Add ARMv8.2-A to TargetParser

[ARM] Add ARMv8.2-A to TargetParser

Add ARMv8.2-A to TargetParser, so that it can be used by the clang
command-line options and the .arch directive.

Most testing of this will be done in clang, checking that the
command-line options that this enables work.

Differential Revision: http://reviews.llvm.org/D15037

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254400 91177308-0d34-0410-b5e6-96231b3b80d8