conformtest: Add x32 XFAILs for mq_attr element types (bug 21279).

POSIX specifies long as the type of elements of struct mq_attr.  For
x32, they are __syscall_slong_t (i.e. long long).  This patch XFAILs
the corresponding tests for x32 in the conformtest expectations (the
bug should not be closed without an actual fix).

Tested with build-many-glibcs.py.

	[BZ #21279]
	* sysdeps/unix/sysv/linux/x86_64/x32/Makefile
	[$(subdir) = conform] (conformtest-xfail-conds): Update comment.
	* conform/data/mqueue.h-data (mq_attr.mq_flags): XFAIL for
	x86_64-x32-linux.
	(mq_attr.mq_maxmsg): Likewise.
	(mq_attr.mq_msgsize): Likewise.
	(mq_attr.mq_curmsgs): Likewise.