Fix swscanf vswscanf namespace (bug 18542).

Fix swscanf vswscanf namespace (bug 18542).

swscanf (added in C90 Amendment 1, present in UNIX98) calls vswscanf
(added in C99, not in C90 Amendment 1 or UNIX98).  This patch fixes
this by using __vswscanf instead and making vswscanf into a weak
alias.

(I intend to add conform/ test support for C90 Amendment 1 - and
various other standard versions supported by glibc but not yet by
conform/ tests - at some point, once the results for currently tested
standards are cleaner.)

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

	[BZ #18542]
	* libio/iovswscanf.c (__vswscanf): Use libc_hidden_def.
	(vswscanf): Use ldbl_weak_alias instead of ldbl_strong_alias
	* include/wchar.h (__vswscanf): Declare.  Use libc_hidden_proto.
	* libio/swscanf.c (__swscanf): Call __vswscanf instead of
	vswscanf.
	* conform/Makefile (test-xfail-UNIX98/wchar.h/linknamespace):
	Remove variable.