rts: Use pthread itimer implementation on Darwin

We want to avoid using SIGALRM whenever possible since we will interrupt
long-running system calls. See #10840.

Test Plan: Validate on Darwin

Reviewers: austin, erikd, simonmar

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2796

GHC Trac Issues: #10840