[AArch64] Add an SVE target feature to the backend and TargetParser.

The feature will be used properly once assembler/disassembler support
begins to land.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307917 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed