Link errors due to forcibly exporting findPtr

The change in https://phabricator.haskell.org/rGHC900c47f88784b91517c00be3e1087322e62f698e is causing link errors due to the combination of -Wl,-u and -Wl,--export-all-symbols

Wrong exit code for T10955dyn()(expected 0 , actual 2 )
Stderr ( T10955dyn ):
E:/ghc-dev/msys64/home/Tamar/ghc/rts/dist/build/libHSrts_thr.a(Printer.thr_o): In function `findPtr':
E:\ghc-dev\msys64\home\Tamar\ghc/rts/Printer.c:880: multiple definition of `findPtr'
./bin_dyn/libA.dll.a(d020776.o):(.text+0x0): first defined here

I believe the correct solution is to just use a cabal flag to emulate the setup we had with make, instead of forcing this symbol always.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions	D5138
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