Avoid creating bg thds for huge arena lone.

For low arena count settings, the huge threshold feature may trigger an unwanted
bg thd creation.  Given that the huge arena does eager purging by default,
bypass bg thd creation when initializing the huge arena.

include/jemalloc/internal/arena_externs.h

@@ -85,6 +85,7 @@ void arena_nthreads_dec(arena_t *arena, bool internal);
 size_t arena_extent_sn_next(arena_t *arena);
 arena_t *arena_new(tsdn_t *tsdn, unsigned ind, extent_hooks_t *extent_hooks);
 bool arena_init_huge(void);
+bool arena_is_huge(unsigned arena_ind);
 arena_t *arena_choose_huge(tsd_t *tsd);
 bin_t *arena_bin_choose_lock(tsdn_t *tsdn, arena_t *arena, szind_t binind,
     unsigned *binshard);