Summary: Because an `int` plays much much nicer with code already written for Windows than `void*` does. It takes quite a bit of hackery to make it possible with the pthread implementation we support using, but it is possible and is worth the effort.

Reviewed By: yfeldblum

Differential Revision: D5276968

fbshipit-source-id: 4cd0d3120a4f19976e76853ef4b337e96d5005a8