--os:ios needs to imply defined(macosx) [backport] (#12585)

(cherry picked from commit 62fc3db9d7)

compiler/options.nim

@@ -409,7 +409,7 @@ proc isDefined*(conf: ConfigRef; symbol: string): bool =
     of "mswindows", "win32": result = conf.target.targetOS == osWindows
     of "macintosh":
       result = conf.target.targetOS in {osMacos, osMacosx, osIos}
-    of "osx":
+    of "osx", "macosx":
       result = conf.target.targetOS in {osMacosx, osIos}
     of "sunos": result = conf.target.targetOS == osSolaris
     of "nintendoswitch":