only test upon spirv targe tenabled

Author: ianayl

mlir/test/Conversion/MathToXeVM/lit.local.cfg

@@ -0,0 +1,7 @@
+spirv_backend_tests = [
+    'native-spirv-builtins.mlir',
+]
+ 
+# Exclude SPIRV backend tests if SPIRV target is disabled: 
+if(not config.run_xevm_tests):
+    config.excludes.update(spirv_backend_tests)