libstats: Improve ABI assertion.

On platforms where pointers are larger than 64-bits, struct statsblob
may be harmlessly padded out such that opaque[] always has some included
space.  Make the assertion more general by comparing to the offset of
opaque rather than the size of struct statsblob.

Discussed with:	jhb, James Clarke
Reviewed by:	trasz, lstewart
Obtained from:	CheriBSD
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D22188

sys/kern/subr_stats.c

@@ -124,7 +124,8 @@ struct statsblobv1 {
 	struct voi	vois[];		/* Array indexed by [voi_id]. */
 } __aligned(sizeof(void *));
 _Static_assert(offsetof(struct statsblobv1, cursz) +
-    SIZEOF_MEMBER(struct statsblobv1, cursz) == sizeof(struct statsblob),
+    SIZEOF_MEMBER(struct statsblobv1, cursz) ==
+    offsetof(struct statsblob, opaque),
     "statsblobv1 ABI mismatch");
 
 struct statsblobv1_tpl {