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.

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -308,6 +308,9 @@
 /* Adaptive mutex support in pthreads. */
 #undef JEMALLOC_HAVE_PTHREAD_MUTEX_ADAPTIVE_NP
 
+/* GNU specific sched_getcpu support */
+#undef JEMALLOC_HAVE_SCHED_GETCPU
+
 /*
  * If defined, jemalloc symbols are not exported (doesn't work when
  * JEMALLOC_PREFIX is not defined).