Display -mcpu/-mattr help even with no source files given.

Unfortunately, we can't easily add a test for this in our
current test runner setup (i.e. dmd-testsuite).

GitHub: Fixes #505.

driver/main.cpp

@@ -202,6 +202,9 @@ static void parseCommandLine(int argc, char **argv, Strings &sourceFiles, bool &
 #endif
     );
 
+    helpOnly = mCPU == "help" ||
+        (std::find(mAttrs.begin(), mAttrs.end(), "help") != mAttrs.end());
+
     // Print config file path if -v was passed
     if (global.params.verbose) {
         const std::string& path = cfg_file.path();