allow specifying rustc when generating target information

[Qix-]

Hello!

Adding a new platform target to rust-lang/rust was resulting in some hard to track errors, but I finally figured out it had to do with the generated list here. Adding them manually to the generated output in a fork is of course possible but I also added a means of passing in a path to rustc, e.g. from one of the bootstrap stages, to automatically update the target list.

Might be useful for those who are trying to get a fork up and running without manually editing generated stuff.

[madsmtm]

I'm mostly indifferent on this, will let @NobodyXu decide.

I do think there's value in the code currently, since it forces users to have +nightly and +$MSRV, which makes it easier for them to see what they need to do to re-generate the same as CI does. So this might also just be solvable with better docs, and the addition of a +stage1 override?

[Qix-]

Having any sort of +override means that you'd have to explicitly link rustup profiles, right?

Thinking about things a bit more, I think the sort of "ideal" case is that, in cases where the compiler is bootstrapping, forking/patching isn't necessary at all, and the target list can be fetched given the bootstrapped compilers at each stage - though that's a drastically more complicated solution.

Either way, I just did this to make things a little easier and to remove human error - I know my use case is niche so it may not be the best way to do it, or even valuable to have. If the +stage1 override or something similar is preferred just let me know and I can adapt :)

[NobodyXu]

Thanks LGTM