Alexander Vershilov authored Nov 22, 2016 and Ben Gamari committed Nov 22, 2016

Allow to unregister threadWaitReadSTM/threadWaitWriteSTM on
a non-threaded runtime. Previosly noop action was returned,
as a result it was not possible to unregister action, unless
data arrives to Fd or it's closed.

Fixes #12852.

Reviewers: simonmar, hvr, austin, bgamari, trofi

Reviewed By: bgamari, trofi

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2729

GHC Trac Issues: #12852