Fix regex wcrtomb namespace (bug 18496).

Fix regex wcrtomb namespace (bug 18496).

The regex code brings in references to wcrtomb, which isn't in all the
standards that contain regex.  This patch makes it call __wcrtomb
instead (in fact some places already called __wcrtomb, so this patch
makes it internally consistent about which name is used).

Tested for x86_64 and x86 that installed stripped shared libraries are
unchanged by the patch.

	[BZ #18496]
	* posix/regex_internal.c (build_wcs_upper_buffer): Call __wcrtomb
	instead of wcrtomb.