[ORC] skip reoptimization tests on s390x. (llvm#112796)

The test was failing on s390x with this error:

JIT session error: Unsupported target machine architecture in ELF object
<main>-jitted-objectbuffer

Author: tstellar

llvm/unittests/ExecutionEngine/Orc/ReOptimizeLayerTest.cpp

@@ -43,6 +43,10 @@ class ReOptimizeLayerTest : public testing::Test {
     if (Triple.isOSBinFormatCOFF() && Triple.isAArch64())
       GTEST_SKIP();
 
+    // SystemZ is not supported yet.
+    if (Triple.isSystemZ())
+      GTEST_SKIP();
+
     if (Triple.isPPC())
       GTEST_SKIP();