[SafeStack] Use proper API to get stack guard

Using the proper API automatically sets `__stack_chk_guard` to `dso_local` if
`Reloc::Static`. This wasn't strictly necessary until recently when dso_local was
no longer implied by `TargetMachine::shouldAssumeDSOLocal` for
`__stack_chk_guard`. By using the proper API, we can avoid generating unnecessary
GOT relocations.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D102646