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

Description

[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

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