[AArch64] Don't crash trying to resolve __stack_chk_guard.

In certain cases, the compiler might try to merge __stack_chk_guard with
another global variable.  (Or someone could theoretically define
__stack_chk_guard as an alias.)  In that case, make sure we don't crash.

Differential Revision: https://reviews.llvm.org/D45746

llvm-svn: 330495