build: Propagate link_language instead of internals

[sp1ritCS]

If a build target has link_language set, use that instead of propagating the language the target was written in.

This fixes the issue, that if you have a build target with cpp sources but with link_language being set to 'c', targets which link against the build target will be linked with the cpp linker, even tho that isn't necessary.

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The non-link_language branch has some special casing for rust, which might have to be replicated in the new branch too. I'm not quite sure what exactly is does, so I'd appreciate some feedback on if that is necessary.

[bonzini]

Yes, I think it should be replicated, and the combined loop could be something like this:

target_langs = [t.link_language] if t.link_language else list(t.compilers.keys())
for lang in target_langs:
    # Rust is always linked through a C-ABI target, so do not add
    # the compiler here
    if lang != 'rust' and lang not in self.compilers:
        self.compilers[lang] = t.all_compilers[lang]

[sp1ritCS]

@bonzini ok, your code was missing the lang in link_langs check, but after adding that it seems to work.

The only problem that still exists is the lacking introspection from the vs backend. Should I just limit the testcase to ninja or is there a better way of detecting the linker a target is going to use?

[bonzini]

Should I just limit the testcase to ninja

Yeah, that makes sense but please add a comment.

[sp1ritCS]

add a comment

I guess the skip reason works well enough for that :)

[dcbaker]

A couple of small nits, otherwise this looks good to me.

[sp1ritCS]

Will re-trigger CI once GitHub actions are back up and running properly again