Rollup merge of #104748 - lqd:download_lld, r=jyn514

Ensure `lld` is supported with `download-ci-llvm`

This PR:
- ensures LLD's step in bootstrap's dist, but it's not strictly necessary since dist will already package it when it's present.
- makes bootstrap's `native::LLD` step support using the packaged `ci-llvm/bin/lld`, instead of building it from source (which would most likely not be available today, nor in the future where `download-ci-llvm = if-available` is the default).

If I understand correctly, `--enable-full-tools` will also enable `rust.lld`, and this is why LLD is already packaged today in the `rust-dev` component on the main targets (and why `-Zgcc-ld=lld` does work there).

That means it's likely that this PR will not be able to land before I've reworked and landed #101792: if LLD is available in `download-ci-llvm`, the `needs-rust-lld` tests should start being executed on the x64 macOS test builders, and CI would fail today.

I've tested locally that building with `download-ci-llvm = true` and `lld = true` with the LLVM submodule unregistered was successful, and that `rust-lld` and the various `lld-wrapper`s are present and `-Zgcc-ld=lld` works as well, on a few different platforms:
- `x86_64-unknown-linux-gnu`
- `aarch64-apple-darwin`
- `x86_64-pc-windows-msvc` (with `-Clinker=rust-lld` rather than `-Zgcc-ld=lld`)
- `x86_64-apple-darwin`, with the `MACOSX_DEPLOYMENT_TARGET` workaround for #101653

I don't think we really need to bump the `download-ci-llvm-stamp` in this case, since `./build/$triple/ci-llvm/bin/lld` is present on all the above targets already, but have added it mechanically, and it should probably be removed to avoid unnecessary downloads/churn.

Fixes #98340
Supersedes #100010

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