Make split sections by default work again
Commit 266a9dc4c changed = to := in one place in mk/config.mk.in.
This broke SupportsSplitSections because the variable LdIsGNULd
that it depends on is not defined until later in the file.
Test Plan: pushed to a wip/ branch for perf to build
Reviewers: DemiMarie, austin, bgamari, mpickering
Reviewed By: mpickering
Subscribers: mpickering, thomie
Differential Revision: https://phabricator.haskell.org/D3070
GHC Trac Issues: Trac #13230