Since x86 has an optimized mempcpy and GCC can inline mempcpy on x86,
define _HAVE_STRING_ARCH_mempcpy to 1 for x86.
[BZ #19759]
* sysdeps/x86/bits/string.h (_HAVE_STRING_ARCH_mempcpy): New.
(cherry picked from commit 2b35e48c)
(cherry picked from commit b4456470)