[asan,test] Disable alloca_loop_unpoisoning.cpp on s390{{.*}}

The test (from https://reviews.llvm.org/D7098) is about the interaction
of VLA and alloca where the VLA causes alloca to have the same address.
This is mostly about behavior checking and less about instrumentation
correctness, so I think it is fair to disable it for a platform that
does not work after StackSafetyAnalysis is enabled by default (#77210).