Arena: share bin offsets in a global.

This saves us a cache miss when lookup up the arena bin offset in a remote
arena during tcache flush.  All arenas share the base offset, and so we don't
need to look it up repeatedly for each arena.  Secondarily, it shaves 288 bytes
off the arena on, e.g., x86-64.

src/ctl.c

@@ -3423,7 +3423,7 @@ stats_mutexes_reset_ctl(tsd_t *tsd, const size_t *mib,
 
 		for (szind_t i = 0; i < SC_NBINS; i++) {
 			for (unsigned j = 0; j < bin_infos[i].n_shards; j++) {
-				bin_t *bin = &arena->bins[i].bin_shards[j];
+				bin_t *bin = arena_get_bin(arena, i, j);
 				MUTEX_PROF_RESET(bin->lock);
 			}
 		}