Fix fnmatch wmemchr namespace (bug 18468).

fnmatch brings in references to wmemchr, which isn't in all the
standards that contain fnmatch, resulting in linknamespace test
failures.  This patch fixes this in the usual way, making wmemchr into
a weak alias for __wmemchr.

Tested for x86_64 and x86 (testsuite, and that disassembly of
installed shared libraries is unchanged by the patch).

	[BZ #18468]
	* wcsmbs/wmemchr.c (wmemchr): Rename to __wmemchr and define as
	weak alias of __wmemchr.  Use libc_hidden_weak.
	* include/wchar.h (__wmemchr): Declare.  Use libc_hidden_proto.
	* posix/fnmatch.c [HANDLE_MULTIBYTE] (MEMCHR): Use __wmemchr
	instead of wmemchr.