Store ncached_max * ptr_size in tcache_bin_info.

With the cache bin metadata switched to pointers, ncached_max is usually
accessed and timed by sizeof(ptr). Store the results in tcache_bin_info for
direct access, and add a helper function for the ncached_max value.

test/unit/cache_bin.c

@@ -10,7 +10,7 @@ TEST_BEGIN(test_cache_bin) {
 
 	assert_ptr_not_null(stack, "Unexpected mallocx failure");
 	/* Initialize to empty; bin 0. */
-	cache_bin_sz_t ncached_max = tcache_bin_info[0].ncached_max;
+	cache_bin_sz_t ncached_max = cache_bin_ncached_max_get(0);
 	void **empty_position = stack + ncached_max;
 	bin->cur_ptr.ptr = empty_position;
 	bin->low_water_position = bin->cur_ptr.lowbits;