[AArch64,Assembler] Compiler support for ID_MMFR5_EL1

Summary: Add read-only system register ID_MMFR5_EL1 and unit tests.

Subscribers: kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69039

llvm-svn: 375010