BUILD/MINOR: threads: unbreak build with threads disabled

BUILD/MINOR: threads: unbreak build with threads disabled

Depending on the optimization level, gcc may complain that wake_thread()
uses an invalid array index for poller_wr_pipe[] when called from
__task_wakeup(). Normally the condition to get there never happens,
but it's simpler to ifdef out this part of the code which is only
used to wake other threads up. No backport is needed, this was brought
by the recent introduction of the ability to wake a sleeping thread.