[ARM] Add v8.1a architecture

Add support for ARMv8.1a 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, rengolin

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D8799

llvm-svn: 237349