rts: Error on invalid --numa flags

Previously things like `+RTS --numa-debug` would enable NUMA support,
despite being an invalid flag.

rts/RtsFlags.c

@@ -944,9 +944,14 @@ error = true;
                       if (rts_argv[arg][6] == '=') {
                           mask = (StgWord)strtol(rts_argv[arg]+7,
                                                  (char **) NULL, 10);
-                      } else {
+                      } else if (rts_argv[arg][6] == '\0'){
                           mask = (StgWord)~0;
+                      } else {
+                          errorBelch("%s: unknown flag", rts_argv[arg]);
+                          error = true;
+                          break;
                       }
+
                       if (!osNumaAvailable()) {
                           errorBelch("%s: OS reports NUMA is not available",
                                      rts_argv[arg]);