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.

configure.ac

@@ -1598,6 +1598,15 @@ if test "x$have_secure_getenv" = "x1" ; then
   AC_DEFINE([JEMALLOC_HAVE_SECURE_GETENV], [ ])
 fi
 
+dnl Check if the GNU-specific sched_getcpu function exists.
+AC_CHECK_FUNC([sched_getcpu],
+              [have_sched_getcpu="1"],
+              [have_sched_getcpu="0"]
+             )
+if test "x$have_sched_getcpu" = "x1" ; then
+  AC_DEFINE([JEMALLOC_HAVE_SCHED_GETCPU], [ ])
+fi
+
 dnl Check if the Solaris/BSD issetugid function exists.
 AC_CHECK_FUNC([issetugid],
               [have_issetugid="1"],