Implement per-CPU arena.

The new feature, opt.percpu_arena, determines thread-arena association
dynamically based CPU id. Three modes are supported: "percpu", "phycpu"
and disabled.

"percpu" uses the current core id (with help from sched_getcpu())
directly as the arena index, while "phycpu" will assign threads on the
same physical CPU to the same arena. In other words, "percpu" means # of
arenas == # of CPUs, while "phycpu" has # of arenas == 1/2 * (# of
CPUs). Note that no runtime check on whether hyper threading is enabled
is added yet.

When enabled, threads will be migrated between arenas when a CPU change
is detected. In the current design, to reduce overhead from reading CPU
id, each arena tracks the thread accessed most recently. When a new
thread comes in, we will read CPU id and update arena if necessary.

src/stats.c

@@ -621,6 +621,7 @@ stats_general_print(void (*write_cb)(void *, const char *), void *cbopaque,
 	OPT_WRITE_BOOL(abort, ",")
 	OPT_WRITE_CHAR_P(dss, ",")
 	OPT_WRITE_UNSIGNED(narenas, ",")
+	OPT_WRITE_CHAR_P(percpu_arena, ",")
 	OPT_WRITE_SSIZE_T_MUTABLE(decay_time, arenas.decay_time, ",")
 	OPT_WRITE_CHAR_P(junk, ",")
 	OPT_WRITE_BOOL(zero, ",")