Fix Windows builds after D1242
Only use the merge_sections.ld linker script if SplitSections is enabled.
I thought I had a way to make the linker script actually work on Windows
and produce object files instead of image files (by using the "INSERT AFTER"
linker script command to get the default script's output format setting),
but that fix caused ghci to crash with an illegal instruction on startup.
Gave up and made a simpler fix of just disabling this for normal builds,
as it's only really relevant with SplitSections enabled anyway.
Reviewers: austin, thomie, bgamari
Reviewed By: austin, thomie
Subscribers: Phyx, thomie
Differential Revision: https://phabricator.haskell.org/D1505
GHC Trac Issues: Trac #8405