rename gc to mm (nim-lang#19971)

Author: ringabout

compiler/msgs.nim

@@ -677,7 +677,7 @@ proc genSuccessX*(conf: ConfigRef) =
   const debugModeHints = "none (DEBUG BUILD, `-d:release` generates faster code)"
   if conf.cmd in cmdBackends:
     if conf.backend != backendJs:
-      build.add "gc: $#; " % $conf.selectedGC
+      build.add "mm: $#; " % $conf.selectedGC
       if optThreads in conf.globalOptions: build.add "threads: on; "
       build.add "opt: "
       if optOptimizeSpeed in conf.options: build.add "speed"