Set soname for cdylibs ( if major version is > 0)

[jschwe]

Since this issue is probably not going to be addressed by cargo in the near future, we should set soname from the corrosion side.
This might require --crate-type support, so a rather recent rustc version could be needed. But since the linker is not invoked for staticlibs, maybe setting the soname could just be done in general for lib targets, that have a cdylib.

[jschwe]

-C link-arg=-Wl,-soname,libmycrate.so

Determining if -Wl, needs to be added may be difficult, since corrosion would need to know the linker driver.
This would have to work with both NO_LINKER_OVERRIDE (rust chooses the linker) and corrosion_set_linker (the user specifies the linker).

If the former option is specified, I don't think we can really do anything. For the latter, we could try to guess the linker flavor or ask the user to specify the linker flavor.