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.