[AArch64] Allow global register asm("x18") or asm("w18") under -ffixed-x18

When using -ffixed-x18, the x18 (or w18) register can safely be used
with the "global register variable" GCC extension, but the backend
fails to recognize it.

Patch by Roland McGrath.

Differential Revision: https://reviews.llvm.org/D31793

llvm-svn: 299799