[mlir] Fix bots after bytecode support was added in D131747

* Fix ambiguous Twine constructor call
* Ensure shift is 64-bit (for MSVC)
* Disable bytecode tests on s390x (we don't support big endian right now)

mlir/lib/Bytecode/Reader/BytecodeReader.cpp

@@ -40,7 +40,7 @@ static std::string toString(bytecode::Section::ID sectionID) {
   case bytecode::Section::kIR:
     return "IR (4)";
   default:
-    return ("Unknown (" + Twine(sectionID) + ")").str();
+    return ("Unknown (" + Twine(static_cast<unsigned>(sectionID)) + ")").str();
   }
 }