x.py dist cross-compile packages the wrong LLVM build

[adamgemmell]

Reproduction, on an x86_64 host:

./x.py dist --host aarch64-unknown-linux-gnu --target aarch64-unknown-linux-gnu
or
./src/ci/docker/run.sh dist-aarch64-linux

Expected:

All packaged artifacts are targeted at aarch64.

Actual:

The LLVM libs in dist/rust-dev-nightly-aarch64-unknown-linux-gnu.tar.xz are x86_64.

More info

The binaries in the same archive are fine.

Building rustc on an aarch64 host with download-ci-llvm = if-available results in linker errors.
Setting this to false and building LLVM locally works fine.

https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/llvm-project.20aarch64.20builds.20corrupt
The oldest CI artifact currently available (https://ci-artifacts.rust-lang.org/rustc-builds/6645da366eed0c61258a04265bea513e94df7ea6/rust-dev-nightly-aarch64-unknown-linux-gnu.tar.gz) has this bug.

It could conceivably affect other distribution artifacts cross-compiled from x86_64 but I haven't tested them.

[adamgemmell]

@rustbot claim