conformtest: Add mips XFAIL for struct stat st_rdev type (bug 21278).

MIPS o32 struct stat has the wrong type of st_rdev.  This patch XFAILs
that test in the conformtest expectations for this case (the bug
should not be closed without an actual fix, however).

Tested with build-many-glibcs.py.

	[BZ #21278]
	* sysdeps/unix/sysv/linux/mips/mips32/Makefile
	[$(subdir) = conform] (conformtest-xfail-conds): Update comment.
	* conform/data/sys/stat.h-data (stat.st_rdev): XFAIL for
	mips-o32-linux.