Bootstrapping on completely new platforms

[crummel]

@uweigand brought up that ./prep.sh --bootstrap does not help for platforms that are not currently supported by Microsoft because no packages exist for those platforms. To work around this, he is manually building a subset of runtime that produces the necessary packages - these are all native bits so they do not require a bootstrap compiler. This is something that we could add to bootstrapping and may even be preferable to the current approach of downloading the Microsoft-built packages.

[MichaelSimons]

[Triage] - @uweigand, @crummel - Can we capture the specific steps (e.g. projects built) taken in this issue? This would make this more actionable for the .NET 8 timeframe.

[uweigand]

Right now, we need e.g. the source-build reference packages before building the runtime, but building the reference packages requires ilasm - which is built as part of the runtime.

Therefore, my specific suggestion would be to add an initial pass of building the runtime, but only the clr.iltools target, before the reference packages (followed by the full runtime build at is usual place afterwards). That would eliminate the need for pre-built ilasm/ildasm packages.