Using unsafe foreign import for rtsSupportsBoundThreads (part of #9696)

A safe import is unnecessary considering rtsSupportsBoundThreads simply
returns a constant.

This commit doesn't fix the main issue of ticket #9696 that
"readRawBufferPtr and writeRawBufferPtr allocate memory".

Reviewers: bgamari, austin, hvr

Reviewed By: hvr

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1964

GHC Trac Issues: #9696