[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, phosek, srhines, nickdesaulniers, efriedma

Reviewed By: nickdesaulniers, efriedma

Subscribers: niravd, jfb, manojgupta, nickdesaulniers, jyknight, efriedma, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D48580

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341706 91177308-0d34-0410-b5e6-96231b3b80d8