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
Reviewed By: bgamari
Subscribers: rwbarton, thomie, carter
GHC Trac Issues: Trac #15051
Differential Revision: https://phabricator.haskell.org/D4916