[AArch64] Refactor AArch64NamedImmMapper to become dependent on subtarget features.

In order to introduce v8.1a-specific entities, Mappers should be aware of SubtargetFeatures available.

This patch introduces refactoring, that will then allow to easily introduce:

- v8.1-specific "pan" PState for PStateMapper (PAN extension)

- v8.1-specific sysregs for SysRegMapper (LOR,VHE extensions)

Reviewers: jmolloy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8496

Patch by Tom Coxon

llvm-svn: 235089