gc.c: force STACK_GROW_DIRECTION for emscripten

The configure script fails to guess the direction on emscripten.
Perhaps this is because of https://github.com/kripken/emscripten/issues/2093

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62410 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

gc.c

@@ -3996,6 +3996,11 @@ init_mark_stack(mark_stack_t *stack)
 #define STACK_END (ec->machine.stack_end)
 #define STACK_LEVEL_MAX (ec->machine.stack_maxsize/sizeof(VALUE))
 
+#ifdef __EMSCRIPTEN__
+#undef STACK_GROW_DIRECTION
+#define STACK_GROW_DIRECTION 1
+#endif
+
 #if STACK_GROW_DIRECTION < 0
 # define STACK_LENGTH  (size_t)(STACK_START - STACK_END)
 #elif STACK_GROW_DIRECTION > 0