[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