[skip ci] Temporarily disable split-sections on Windows.
Summary: This temporarily disabled split-sections again on Windows because of the overhead in linking it introduces. Unfortunately because BFD is so slow a testsuite run gets almost 2x slower. Simply linking Hello World takes an unacceptable long time. So for now, it'll be disabled as we look into different linkers such as LLD. Test Plan: ./validate Reviewers: austin, bgamari Subscribers: rwbarton, thomie GHC Trac Issues: #12913 Differential Revision: https://phabricator.haskell.org/D3731