[AArch64] Support adding X[8-15,18] registers as CSRs.

Summary:
Specifying X[8-15,18] registers as callee-saved is used to support
CONFIG_ARM64_LSE_ATOMICS in Linux kernel. As part of this patch we:
- use custom CSR list/mask when user specifies custom CSRs
- update Machine Register Info's list of CSRs with additional custom CSRs in
LowerCall and LowerFormalArguments.

Reviewers: srhines, nickdesaulniers, efriedma, javed.absar

Reviewed By: nickdesaulniers

Subscribers: kristof.beyls, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D52216

llvm-svn: 342824