use `minicore` in some `run-make` tests r? jieyouxu This manually builds `minicore` in two more `rmake` tests that rolled their own before, and adds a helper for the path of the minicore source. I tried adding a true minicore helper in `run_make_support` but unfortunately the minicore build needs to inherit some (but probably not all) arguments of the final rustc invocation (notably `target` and `target_cpu`), so a one-size-fits-all helper doesn't really work as far as I can see. For now with 3 uses this is probably fine, but there are probably other `run-make` tests that could use `minicore` but didn't document/simulate that. follow-up to discussion in https://github.com/rust-lang/rust/pull/153153.