global_data: Remove environment members if not used

If the environment is not enabled we don't need these fields in
global_data. Make them conditional.

Make these fields conditional. Move env_buf up one so it can share
an #ifdef.

Signed-off-by: Simon Glass <sjg@chromium.org>

lib/asm-offsets.c

@@ -44,7 +44,9 @@ int main(void)
 
 	DEFINE(GD_NEW_GD, offsetof(struct global_data, new_gd));
 
+#if CONFIG_IS_ENABLED(ENV_SUPPORT)
 	DEFINE(GD_ENV_ADDR, offsetof(struct global_data, env_addr));
+#endif
 
 	return 0;
 }