src: reduce cpu profiler overhead

Reduce the overhead of the CPU profiler by suppressing SIGPROF signals
when sleeping / polling for events. Avoids unnecessary wakeups when the
CPU profiler is active. Depends on https://github.com/libuv/libuv#15.

Ref: https://github.com/strongloop/strong-agent/issues/3
PR-URL: https://github.com/joyent/node/pull/8791
Reviewed-by: Trevor Norris <trev.norris@gmail.com>