Merge objects broken when `-L` is not supported (fallback case using ar)

It appears that attempting to created a shared library is broken in the case when using ar rather than ld -r in joinObjectFiles.

In particular this is observed when a module has a foreign export, the resulting shared object doesn't contain a symbol for the foreign export definition.

I would like to add a test for this, perhaps the easiest way (which doesn't require nm) is to attempt to link a C file which references the foreign export to the created shared library.

There is a cabal test for this, which has been broken since ghc-9.4, but only on windows as other platforms support merging objects via ld -r.

The GHC tests are not sufficient as they do not check the created shared object has the correct symbols.