[GlobalISel][Test] Fix IRTranslatorBF16Test crash (llvm#188273)

Skip the test when the AArch64 target is unavailable.

Author: lialan

llvm/unittests/CodeGen/GlobalISel/IRTranslatorBF16Test.cpp

@@ -87,6 +87,8 @@ TEST_F(AArch64IRTranslatorTest, IRTranslateBfloat16) {
   )");
 
   auto TM = createTargetMachine();
+  if (!TM)
+    GTEST_SKIP();
   M->setDataLayout(TM->createDataLayout());
 
   TM->setGlobalISel(true);