rts: A bit of cleanup of posix itimer implementation
rts: A bit of cleanup of posix itimer implementation

* Use bool instead of HsBool
* Use barf instead of sysErrorBelch; stg_exit

Test Plan: Validate

Reviewers: erikd, simonmar

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4874

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc