soxstack: slience 32-bit gcc warning

Modern GCC complains when casting pointers to integers of a different
size (even a larger one).  Switch from uint64_t to uintptr_t which will
always be the right size for a stack address instead of maybe being too
big.

Reviewed by:	dchagin, emaste
Differential Revision:	https://reviews.freebsd.org/D45087

tests/sys/vm/soxstack/soxstack.c

@@ -23,7 +23,7 @@ checkstack(void)
 	struct kinfo_vmentry *freep, *kve;
 	struct kinfo_proc *p;
 	struct procstat *prstat;
-	uint64_t stack;
+	uintptr_t stack;
 	int i, cnt;
 
 	prstat = procstat_open_sysctl();
@@ -33,7 +33,7 @@ checkstack(void)
 	freep = procstat_getvmmap(prstat, p, &cnt);
 	assert(freep != NULL);
 
-	stack = (uint64_t)&i;
+	stack = (uintptr_t)&i;
 	for (i = 0; i < cnt; i++) {
 		kve = &freep[i];
 		if (stack < kve->kve_start || stack > kve->kve_end)