rts: fix unused parameter warning

Summary:
If `pthread_setname_np` is not available, then a regular
./validate will fail due to warnings; the `name` parameter to
`createOSThread` becomes unused.

Signed-off-by: Austin Seipp <austin@well-typed.com>

Test Plan: iiam

Reviewers: simonmar, nomeata, jstolarek, hvr

Reviewed By: nomeata, jstolarek, hvr

Subscribers: nomeata, thomie, carter, ezyang, simonmar

Differential Revision: https://phabricator.haskell.org/D344