Summary:
Initial implementation of split-section on Windows.

This also corrects section namings and uses the platform
convention of `$` instead of `.` to separate sections.

Implementation is based on @awson's patches to binutils.

Binutils requires some extra help when compiling the libraries
for GHCi usage. We drop the `-T` and use implicit scripts to amend
the linker scripts instead of replacing it.

Because of these very large GHCi object files, we need big-obj support,
which will be added by another patch.

Test Plan: ./validate

Reviewers: awson, austin, bgamari

Subscribers: dfeuer, rwbarton, thomie, snowleopard, #ghc_windows_task_force

GHC Trac Issues: #12913

Differential Revision: https://phabricator.haskell.org/D3383