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

Summary:
Making X[8-15,18] registers call-saved is used to support
CONFIG_ARM64_LSE_ATOMICS in Linux kernel.

Signed-off-by: Tri Vo <trong@android.com>

Reviewers: srhines, nickdesaulniers, javed.absar

Reviewed By: nickdesaulniers

Subscribers: kristof.beyls, jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D52399

llvm-svn: 342990