Summary:
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: #8405