Raise MAXDSIZ from 8G to 32G. The old limit was just an arbitary choice

that was greater than 4G.  I originally used the same values as i386 in
order to save opening a new PML4 page slot, but in the day of gigabytes
of memory, worrying about a 4K page seems futile.  Moving from 8 to 32G
moves the page to a different index, it doesn't increase the number of
pages used.

sys/amd64/include/vmparam.h

@@ -57,7 +57,7 @@
 #define	DFLDSIZ		(128UL*1024*1024)	/* initial data size limit */
 #endif
 #ifndef MAXDSIZ
-#define	MAXDSIZ		(8192UL*1024*1024)	/* max data size */
+#define	MAXDSIZ		(32768UL*1024*1024)	/* max data size */
 #endif
 #ifndef	DFLSSIZ
 #define	DFLSSIZ		(8UL*1024*1024)		/* initial stack size limit */