[AArch64] Rename v8.1a from "extension" to "architecture"

v8.1a is renamed to architecture, accordingly to approaches in ARM backend.

Excess generic cpu is removed. Intended use: "generic" cpu with "v8.1a" subtarget feature

Reviewers: jmolloy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8766

llvm-svn: 233810