Fix remaining return type of ifunc resolver declaration

Fix remaining return type of ifunc resolver declaration

Since Martin Sebor's commit

commit ee4e992ebe5f9712faedeefe8958b67d61eaa0f2
Author: Martin Sebor <msebor@redhat.com>
Date:   Tue Aug 22 09:35:23 2017 -0600

    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.

builds for powerpc64le fail in the declaration of some ifunc resolvers,
because the ifunc is declared with unmatching return types.  One of the
declarations comes from the __ifunc_resolver macro, which was patched by
the aforementioned commit:

    /* Helper / base  macros for indirect function symbols.  */
    #define __ifunc_resolver(type_name, name, expr, arg, init, classifier) \
      classifier inhibit_stack_protector                                   \
      __typeof (type_name) *name##_ifunc (arg)                             \

whereas the other comes from the unpatched __ifunc macro when
HAVE_GCC_IFUNC is not defined:

    # define __ifunc(type_name, name, expr, arg, init)                     \
      extern __typeof (type_name) name;                                    \
      void *name##_ifunc (arg) __asm__ (#name);                            \

This patch changes the return type of the ifunc resolver in the __ifunc
macro, so that it matches the return type of the target function,
similarly to what the aforementioned commit does.

Tested for powerpc64le and s390x with unpatched GCC.

	* include/libc-symbols.h: [!defined HAVE_GCC_IFUNC] (__ifunc):
	Change the return type of the ifunc resolver to match the return
	type of the target function.