Fix fmtmsg addseverity namespace (bug 18539).

Fix fmtmsg addseverity namespace (bug 18539).

Use of fmtmsg (XSI POSIX) brings in addseverity (non-POSIX).  This
patch fixes this by making addseverity into a weak alias for
__addseverity.

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

	[BZ #18539]
	* stdlib/fmtmsg.c (addseverity): Rename to __addseverity and
	define as weak alias of __addseverity.
	* conform/Makefile (test-xfail-XPG4/fmtmsg.h/linknamespace):
	Remove variable.
	(test-xfail-UNIX98/fmtmsg.h/linknamespace): Likewise.
	(test-xfail-XOPEN2K/fmtmsg.h/linknamespace): Likewise.
	(test-xfail-XOPEN2K8/fmtmsg.h/linknamespace): Likewise.