Introduce prototype-style function definition for generic stpcpy.

Upstream part of:

commit 142e0a9953059bcf5667921f2670efec377be3d5

Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>

Date:   Tue Jul 28 17:12:25 2015 -0300

    powerpc: Use default strcpy optimization for POWER7

diff --git a/string/stpcpy.c b/string/stpcpy.c

index b74f6e2b489e1f18..8b2c4c5bb7e1ee74 100644

--- a/string/stpcpy.c

+++ b/string/stpcpy.c

@@ -36,9 +36,7 @@ extern __typeof (__stpcpy) STPCPY;

 /* Copy SRC to DEST, returning the address of the terminating '\0' in DEST.  */

 char *

-__stpcpy (dest, src)

-     char *dest;

-     const char *src;

+__stpcpy (char *dest, const char *src)

 {

   char *d = dest;

   const char *s = src;