INIT/cc.darwin: default to -Os (small code) like Apple does

(cherry picked from commit 2210ff9026ca3f05dc88988c72f56eea195753cc)

src/cmd/INIT/cc.darwin

@@ -27,7 +27,7 @@ do	case $op in
 		;;
 	-lm)	continue
 		;;
-	-O)	continue
+	-O)	arg='-Os'  # default to small code: Apple compiles its /bin/ksh with -Os
 		;;
 	esac
 	set "$@" "$arg"