Declare ifunc resolver to return a pointer to the same type as the target

function to help GCC detect incompatibilities between the two when it's
enhanced to do so.

ChangeLog

@@ -1,3 +1,8 @@
+2017-08-22  Martin Sebor  <msebor@redhat.com>
+
+	* include/libc-symbols.h (__ifunc_resolver): Declare resolver
+	to return a pointer to the same type as the target function.
+
 2017-08-22  H.J. Lu  <hongjiu.lu@intel.com>
 
 	[BZ #18822]

include/libc-symbols.h

@@ -803,7 +803,8 @@ for linking")
 
 /* Helper / base  macros for indirect function symbols.  */
 #define __ifunc_resolver(type_name, name, expr, arg, init, classifier)	\
-  classifier inhibit_stack_protector void *name##_ifunc (arg)					\
+  classifier inhibit_stack_protector					\
+  __typeof (type_name) *name##_ifunc (arg)				\
   {									\
     init ();								\
     __typeof (type_name) *res = expr;					\