Fix locale memmem namespace (bug 17585).

Fix locale memmem namespace (bug 17585).

Locale code, brought in by ISO C functions, calls memmem, which is not
an ISO C function.  This isn't an ISO C conformance bug, because all
mem* names are reserved, but glibc practice is not to rely on that
reservation (thus, memmem is only declared in string.h if __USE_GNU
even though ISO C would allow it to be declared unconditionally, for
example).  This patch changes that code to use __memmem.

Note: there are uses of memmem elsewhere in glibc that I didn't
change, although it may turn out some of those also need to use
__memmem.

Tested for x86_64 (testsuite, and that disassembly of installed shared
libraries is unchanged by this patch).

	[BZ #17585]
	* string/memmem.c [!_LIBC] (__memmem): Define to memmem.
	(memmem): Rename to __memmem and define as weak alias of
	__memmem.  Use libc_hidden_weak.
	(__memmem): Use libc_hidden_def.
	* include/string.h (__memmem): Declare.  Use libc_hidden_proto.
	* locale/findlocale.c (valid_locale_name): Use __memmem instead of
	memmem.