[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).

Author: MaskRay

compiler-rt/test/asan/TestCases/alloca_loop_unpoisoning.cpp

@@ -2,6 +2,7 @@
 // RUN: %env_asan_opts=detect_stack_use_after_return=0 %run %t 2>&1
 //
 // REQUIRES: stable-runtime
+// UNSUPPORTED: target=s390{{.*}}
 
 // This testcase checks that allocas and VLAs inside loop are correctly unpoisoned.