[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

mk/config.mk.in

@@ -357,8 +357,9 @@ SplitObjs = $(if $(and $(filter YES,$(SupportsSplitObjs)),\
 # Set SplitSections=YES or NO in your build.mk to override the default.
 #
 # This is not supported on Darwin (where you can use subsections-via-symbols
-# instead) and Windows is not yet working. (See #11445 and related tickets.)
-OsSupportsSplitSections=$(if $(filter $(TargetOS_CPP),darwin),NO,YES)
+# instead) and Windows is disabled until we figure the linking performance
+# issues related to BFD out. (See #11445, #12913 and related tickets.)
+OsSupportsSplitSections=$(if $(filter $(TargetOS_CPP),mingw32 darwin),NO,YES)
 SupportsSplitSections=$(if $(and $(filter YES,$(OsSupportsSplitSections)),\
                                    $(filter YES,$(LdIsGNULd))),YES,NO)
 SplitSections ?= $(SupportsSplitSections)