[AArch64] Don't store available subtarget features in AArch64SysReg::SysRegMapper

Subtarget features must not be a part of the target machine. So, they are now not being stored in SysRegMapper, but provided each time fromString()/toString() are called

Reviewers: jmolloy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8655

llvm-svn: 233386