Work around GHC JS backend not producing .o files for backpack signatures
GHC's JavaScript backend doesn't produce .o files for backpack signature module instantiations, causing ar to fail when creating library archives. Fix this by replacing the ar command in GHC's settings with a wrapper that filters out missing .o files from both command-line arguments and response files. The fix only applies to GHCJS instantiated backpack builds, leaving all other builds unchanged.