Fix build-dependencies resolution when cross-compiling

[pnmadelaine]

This fixes some errors in the resolution of build-dependencies when cross-compiling. All dependencies were computed using the host platform, even build-dependencies which are then compiled for the build platform. I added a minimized example that highlights the issue.

[Ericson2314]

Oh this looks very nice!

[pnmadelaine]

@Ericson2314 I changed a few things since the opening of the PR: the diff is now smaller and the changes are clearer IMO. I do add a name field to the output of makeDefaultTarget though, maybe that's not desirable? I don't know how close the output should match the content of a rustc target specification file. Of course I can always split back target into target and targetSpec (or some better names) like I was doing in the first place. Anyway, written this way I think the change in the logic appears more clearly. Please tell me if you'd prefer having it split!

[Ericson2314]

@alyssais you have any thoughts on the name field? I forgot what Nixpkgs is doing atm, but I do think we want to keep the target stuff in sync.

[alyssais]

Using rustcTarget as this does seems like the right thing.

[Ericson2314]

OK I see what is going on. This would technically allow cfg(name = "x86_64-unknown-gnu-linux") which is not standard, but I think something else will catch that earlier so it's fine. We can rework it if the bug shows up in practice.