Support JSON target specs 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/cargo/blob/592058c7ce08a2ba2628b01e7dc4ce1e72b6bdff/src/cargo/core/compiler/compile_kind.rs#L144-L154).

I have only tested building the compiler and a few tools like rustdoc. I
have not tested doing things like building other tools, running tests,
etc.

This would be much easier if there was a Docker image for testing the
use case of building rustc with a custom target spec (and even better if
that ran in CI).

After the next beta branch, using target JSON specs will become more
cumbersome because target specs with the `.json` extension will now
require passing `-Zjson-target-spec` (from
https://github.com/rust-lang/cargo/pull/16557). This does not affect
target specs without the `.json` extension (such as those from
RUST_TARGET_PATH). From my testing, it should be sufficient to pass
`CARGOFLAGS_NOT_BOOTSTRAP="-Zjson-target-spec"`. I think that should be
fine, since this is not a particularly common use case AFAIK. We could
extend bootstrap to auto-detect if the target is a file path, and pass
`-Zjson-target-spec` appropriately. I tried something similar in
https://github.com/ehuss/rust/commit/f0bdd354835a0993febaf1de214cda2abd9a8500,
which could be adapted if desired.

It would be nice if all of this is documented somewhere.
https://rustc-dev-guide.rust-lang.org/building/new-target.html does not
really say how to build the compiler with a custom json target.

Fixes https://github.com/rust-lang/rust/issues/151729