Added GC exports, so Stubs target will generate them.

lib/mac/NSStdLib/NSStdLib.exp

@@ -992,3 +992,12 @@ scalbn
 #longjmp
 #__terminate
 #__initialize
+
+# GC exports.
+GC_add_roots
+GC_remove_roots
+GC_clear_roots
+GC_generic_init_threads
+GC_gcollect
+GC_stdout
+GC_stderr