[AArch64] Add v8.1a architecture

Add support for AArch64 v8.1 architecture. Briefly it is described on http://community.arm.com/groups/processors/blog/2014/12/02/the-armv8-a-architecture-and-its-ongoing-development

Reviewers: jmolloy

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D8493

llvm-svn: 235110