Add support for assembly language for foreign_stubs

[samoht]

trafficstars

The doc for foreign_stubs says:

language specifies the source language, where c means C and cxx means C++. In future, more languages may be supported.

It would be nice to add support for .S files too ; generally they can just be passed to the underlying C compiler just fine ; I'm not fully sure of what should be the assembly flags. I think keeping the flags as C by default should generally work.

[ghost]

Indeed. That requires #3346

[rgrinberg]

@gridbugs do you think this is still worth doing given that we have custom objects?

[gridbugs]

I don't think this is worth doing anymore as one can now compile the assembly files to object files with a custom rule and then include them with (extra_objects ...). For posterity, the relevant part of the documentation is https://dune.readthedocs.io/en/stable/concepts.html?highlight=extra_objects#extra-objects-1