[AArch64] Add support for -fzero-call-used-regs

Support the "-fzero-call-used-regs" option on AArch64. This involves much less
specialized code than the X86 version. Most of the checks can be done with
TableGen.

Reviewed By: nickdesaulniers, MaskRay

Differential Revision: https://reviews.llvm.org/D124836