Fixed build on OS X (fixes #9684)

Summary:
This commit disables setting a thread name on OS X, since it is not possible to
set it for any other thread than the current one and pthread_setname_np
accepts only a single argument on that platform.

There is an alternative approach I mentioned in the description of the ticket
[1], however it seems to be hard to implement it properly.

1: https://ghc.haskell.org/trac/ghc/ticket/9684

Reviewers: simonmar, hvr, austin

Subscribers: thomie, carter, ezyang, simonmar, mlen

Differential Revision: https://phabricator.haskell.org/D338

GHC Trac Issues: #9684