Double-check `rustc` flag handling in bootstrap

JSON target specs were destabilized in https://github.com/rust-lang/rust/pull/150151 and https://github.com/rust-lang/rust/pull/151534. However, this broke trying to build rustc itself with a JSON target spec. This is because in a few places bootstrap is manually calling rustc without the ability for the user to provide additional flags (primarily, -Zunstable-options to enable JSON targets).

There's a few different ways to fix this. One would be to change these calls to rustc to include flags provided by the user (such as RUSTFLAGS_NOT_BOOTSTRAP). Just to keep things simple, this PR proposes to just unconditionally pass -Zunstable-options.

Another consideration here is how maintainable this is. A possible improvement here would be to have a function somewhere (BootstrapCommand, TargetSelection, free function) that would handle appropriately adding the --target flag. For example, that's what cargo does in CompileKind::add_target_arg.

-- https://github.com/rust-lang/rust/pull/152677

E-hard because this requires a lot of digging without mentoring bandwidth.

Start by inspecting the bootstrap code paths that invoke rustc, then read the referenced PRs 150151, 151534, and 152677 to understand the JSON target regression. Trace how user-provided flags and --target are handled, and verify that building rustc with a JSON target spec works while the relevant bootstrap checks pass.