[mlir] Use the process (host) triple in MLIRTargetLLVMTests (#69538)

The test is meant to execute on the native target and only initializes
the native target. However, it then gets the default target triple
instead of the process (host) triple. This fails in cases where the
native target and the default target are not the same.

The test was added here: https://reviews.llvm.org/D154100

Author: sstamenova

mlir/unittests/Target/LLVM/SerializeToLLVMBitcode.cpp

@@ -55,7 +55,7 @@ TEST_F(MLIRTargetLLVM, SKIP_WITHOUT_NATIVE(SerializeToLLVMBitcode)) {
   ASSERT_TRUE(!!module);
 
   // Serialize the module.
-  std::string targetTriple = llvm::sys::getDefaultTargetTriple();
+  std::string targetTriple = llvm::sys::getProcessTriple();
   LLVM::ModuleToObject serializer(*(module->getOperation()), targetTriple, "",
                                   "");
   std::optional<SmallVector<char, 0>> serializedModule = serializer.run();