Miri tests: use xargo to build separate libstd
This uses `cargo miri setup` to prepare the libstd that is used for testing Miri, instead of adjusting the entire bootstrap process to make sure the libstd that already gets built is fit for Miri.
The issue with our current approach is that with `test-miri = true`, libstd and the test suite get built with `--cfg miri`, which e.g. means hashbrown uses no SIMD, and not all things are tested. Such global side-effects seem like footguns waiting to go off.
On the other hand, the new approach means we install xargo as a side-effect of doing `./x.py test src/tools/miri`, which might be surprising, and we also both have to build xargo and another libstd which costs some extra time. Not sure if the tools builders have enough time budget for that. Maybe there is a way to cache xargo?
We have to first first land https://github.com/rust-lang/miri/pull/870 in Miri and then update this PR to include that change (also to get CI to test Miri before bors), but I wanted to get the review started here.
Cc @oli-obk (for Miri) @alexcrichton (for CI) @Mark-Simulacrum (for bootstrap)
Fixes https://github.com/rust-lang/rust/issues/61833, fixes https://github.com/rust-lang/rust/issues/63219
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