Clean up more fallout from r252976.

libatomic/ChangeLog:
2017-10-02  Martin Sebor  <msebor@redhat.com>

	PR c/81854
	* acinclude.m4 (LIBAT_CHECK_IFUNC): Have ifunc resolver return
	a function pointer rather than void* to avoid GCC 8 warnings.
	* configure: Regenerate.
	* libatomic_i.h: Declare ifunc resolvers to return function
	pointers rather than void*.

From-SVN: r253372