conformtest: XFAIL uc_sigmask test for ia64 (bug 21634).

This patch XFAILs one test where the ia64 ucontext_t has the wrong
type of a field, to allow the conform/ tests as a whole to pass once
the namespace issues are fixed.

Tested with build-many-glibcs.py.

	[BZ #21634]
	* sysdeps/unix/sysv/linux/ia64/Makefile [$(subdir) = conform]
	(conformtest-xfail-conds): New variable.
	* conform/data/signal.h-data (uc_sigmask): XFAIL for ia64-linux.
	* conform/data/ucontext.h-data (uc_sigmask): Likewise.