[llvm] SystemZ: Remove override of insertSSPDeclarations (PR #163708)

[Ulrich Weigand via llvm-commits]

uweigand wrote:

> > Given that, does it make sense to add this SystemZ special case in RuntimeLibcalls.td at this point?
> 
> Yes, RuntimeLibcalls ideally would be accurate for the set of calls that the target could possibly emit

Well, that was my point: once the above patch is merged, we *can* possibly emit the `__stack_chk_guard` call just like other Linux targets, even though we don't by default (again, just like other Linux targets).  There shouldn't be anything special about SystemZ at that point any more.

https://github.com/llvm/llvm-project/pull/163708