Reject running `compiletest` self-tests against stage 0 rustc unless explicitly allowed
Currently, in `pr-check-1`, we run `python3 ../x.py test --stage 0 src/tools/compiletest`, which is `compiletest` self-tests against stage 0 rustc. This makes it very annoying for PRs wanting to change target spec JSON format, which `compiletest` depends on for target information, as otherwise `compiletest` would have to know how to handle 2 different target spec JSON formats and know when to pick which.
Instead of doing that, we change `compiletest` self-tests to reject running against stage 0 `rustc` *unless* explicitly allowed with `build.compiletest-allow-stage0=true`. `build.compiletest-allow-stage0` is a proper bootstrap config option in favor of the ad-hoc `COMPILETEST_FORCE_STAGE0` env var. This means that:
- `./x test src/tools/compiletest --stage=0` is not allowed, unless `build.compiletest-allow-stage0=true` is set. In this scenario, `compiletest` self-tests should be expected to fail unless the stage 0 `rustc` as provided is like codegen_cranelift where it's *actually* built from in-tree `rustc` sources.
- In CI, we change `./x test src/tools/compiletest --stage=0` to `./x test src/tools/compiletest --stage=1`, and move it to `pr-check-2`. Yes, this involves building the stage 1 compiler, but `pr-check-2` already has to build stage 1 compiler to test stage 1 library crates.
- Crucially, this means that **`compiletest` is only intended to support one target spec JSON format**, namely the one corresponding to the in-tree `rustc`.
- This should preserve the `compiletest-use-stage0-libtest` UX optimization where changing `compiler/` tree should still not require rebuilding `compiletest` as long as `build.compiletest-use-stage0-libtest=true`, as that should remain orthogonal. This is completely unlike my previous attempt at https://github.com/rust-lang/rust/pull/144563 that tries to do a way more invasive change which would cause the rebuild problem.
Best reviewed commit-by-commit.
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r? `@Kobzol`
202d1f5bf4