malloc: Fix i686 build from malloc interpose backport

This patch fixes i686 build after malloc interpose fix backport
(commit 749c94d4).

Checked on i686-linux-gnu.

	* malloc/arena.c (__malloc_fork_lock_parent): Add internal_function
	attribute.
	(__malloc_fork_unlock_parent): Likewise.
	(__malloc_fork_unlock_child): Likewise.