[AArch64, ARM] Add v8.1a architecture and generic cpu

New architecture and cpu added, following http://community.arm.com/groups/processors/blog/2014/12/02/the-armv8-a-architecture-and-its-ongoing-development

Reviewers: t.p.northover

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8505

llvm-svn: 233290