function-section: enable on windows

Authored by Phyx on Aug 21 2018, 11:13 AM.

Description

function-section: enable on windows

gc-sections was onced observed to be slow on Windows, which is the only
reason it's not enabled yet. However, it seems to be better now.

Test Plan: ./validate

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: Trac #15051

Differential Revision: https://phabricator.haskell.org/D4916