With this commit, "ocamlc -c -linkall" or "ocamlopt -c -linkall" produce object files that have the "force link" flag set.  Once put in library files (.cma/.cmxa), these object files will always be linked when the library is linked, even if no definition from the object file is referenced.  However, other object files in the library can still be removed if none of their definitions are referenced.

In this respect, we get finer-grained control on which object files should always be linked, typically because they contain important initialization code.

This was a regression in 4.04.0, where passing files with unknown
extension (foo.xwds) to the compiler raises an uncaught Arg.Bad
exception.