• fix gc_thread related compilation failure on Solaris/i386 platform

Summary:
This patch fixes gc_thread related compilation failure
on Solaris/i386 platform. It uses Linux way of __thread declared
gc_thread variable for register starving i386 from now.

Reviewers: bgamari, austin, erikd

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1688