Determine rtree levels at compile time.

Rather than dynamically building a table to aid per level computations,
define a constant table at compile time.  Omit both high and low
insignificant bits.  Use one to three tree levels, depending on the
number of significant bits.

src/extent.c

@@ -1522,8 +1522,7 @@ extent_merge_wrapper(tsdn_t *tsdn, arena_t *arena,
 
 bool
 extent_boot(void) {
-	if (rtree_new(&extents_rtree, (unsigned)((ZU(1) << (LG_SIZEOF_PTR+3)) -
-	    LG_PAGE))) {
+	if (rtree_new(&extents_rtree)) {
 		return true;
 	}