amd64: report support for 1G superpages in getpagesizes(2).

Reviewed by:	markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D24652

sys/amd64/amd64/pmap.c

@@ -2382,6 +2382,11 @@ pmap_init(void)
 		KASSERT(MAXPAGESIZES > 1 && pagesizes[1] == 0,
 		    ("pmap_init: can't assign to pagesizes[1]"));
 		pagesizes[1] = NBPDR;
+		if ((amd_feature & AMDID_PAGE1GB) != 0) {
+			KASSERT(MAXPAGESIZES > 1 && pagesizes[2] == 0,
+			    ("pmap_init: can't assign to pagesizes[2]"));
+			pagesizes[2] = NBPDP;
+		}
 	}
 
 	/*