[AArch64] Support reserving x1-7 registers.

Summary: Reserving registers x1-7 is used to support CONFIG_ARM64_LSE_ATOMICS in Linux kernel. This change adds support for reserving registers x1 through x7.

Reviewers: javed.absar, efriedma, nickdesaulniers, srhines, phosek

Reviewed By: nickdesaulniers

Subscribers: manojgupta, jfb, cfe-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D48581

llvm-svn: 342100