Add a stage0 stdtest build to make check

[brson]

It would be nice to ensure that this continues to work since it makes runtime hacking easier.

[toddaaro]

Some thoughts from my day without stage0 working on the runtime:

The main issue is that with a borked runtime you can't build the stage1 compiler, and then can't run tests as part of stage1. A joy of self-hosting.

A workaround suggested by @brson is to build the stage1 compiler with a clean repository, and then use NO_REBUILD=1 with your changes to skip compiling the compiler. The problem is that this doesn't seem to work - something about this process results in changes to struct definitions causing random segfaults. It is likely other types of changes can cause badtimes too.

So I think the conclusion is that without check-stage0-std we just can't do any runtime development, unless we go through absurd hurdles to debug using rustc as the test program. So even if it means bot cycle time goes up I think it is super important this gets checked.

[bblum]

#8470

[brson]

Closing dupe.