Fix sys/stat.h fchmod namespace (bug 20073).

sys/stat.h declares fchmod if __USE_POSIX (i.e. POSIX.1:1990).  But it
was actually added in 1993 and also in XPG4.  This patch changes the
conditions to the correct __USE_POSIX199309 || __USE_XOPEN_EXTENDED.

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

	[BZ #20073]
	* io/sys/stat.h (fchmod): Declare for
	[__USE_POSIX199309 || __USE_XOPEN_EXTENDED], not [__USE_POSIX].
	* conform/Makefile (test-xfail-XPG3/sys/stat.h/conform): Remove
	variable.