Add Docker image to run AArch64 Linux tests

[emilyalbini]

This PR adds a Docker image to run the AArch64 Linux test suite on a native AArch64 host platform, which will be used in the future to run the test suite in our CI. The image will also be useful for ARM folks to ensure internally that the bugfixes they submit work.

This will be the first Docker image designed to run on a non-x86_64 host platform, and to prevent surprising behavior this PR moves all images requiring a x86_64 host in the src/ci/docker/host-x86_64 directory. Paths and scripts are changed accordingly, and a helpful error message is added when someone tries to run an image on the wrong architecture:

Invalid image: aarch64-gnu
Note: the image exists for the aarch64 host architecture
Note: the current host architecture is x86_64

The old emulated and disabled aarch64-gnu builder is also removed in this PR.

This PR is best reviewed commit-by-commit.
r? @Mark-Simulacrum

[Mark-Simulacrum]

I'm thinking we might want to make sure that the sccache for aarch64 and x86_64 is always of the same version -- just to avoid potential collisions that aren't properly handled or whatever.

[emilyalbini]

Hmm, what do you have in mind to check that?

[Mark-Simulacrum]

Oh I just meant that we'd always have the same date for both -- not automatically checking it, but manually enforcing by updating both at the same time.

[emilyalbini]

We're not running this builder on CI yet, so there is not going to be duplication at least for a little while. Would you be ok if I follow up on sccache in a separate PR? Building it on musl is kinda of a pain, so I'm working on a script.

[Mark-Simulacrum]

Modulo the one nit, seems good to me. r=me when ready and nit is resolved

[emilyalbini]

@bors r=Mark-Simulacrum p=1 rollup=never

[bors]

📌 Commit 0c3661e58d81c7bc9170b09f2550131ce9a40cf8 has been approved by Mark-Simulacrum

[emilyalbini]

This will cause a rebuild of most of the CI images.

[bors]

⌛ Testing commit 0c3661e58d81c7bc9170b09f2550131ce9a40cf8 with merge 48c74ee23c8fc4714b9b7330c8272f380c130d1c...

[bors]

💔 Test failed - checks-azure

[bors]

☔ The latest upstream changes (presumably #73779) made this pull request unmergeable. Please resolve the merge conflicts.

[emilyalbini]

Fixed the COPYs I forgot to change.

@bors r=Mark-Simulacrum p=1

[bors]

📌 Commit f7ed57fa3931894390f217a0077fb43d31454667 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit f7ed57fa3931894390f217a0077fb43d31454667 with merge 3fa5b9a7632cff23d9aa5d22dce447ef9d34ecf3...

[bors]

💔 Test failed - checks-azure

[bors]

💥 Test timed out

[emilyalbini]

@bors retry p=1000

[bors]

⌛ Testing commit 775eeb3 with merge 3d9c4481de234870b4aedee630113c7df21d8566...

[emilyalbini]

The timeout was expected, as this PR had to rebuild the Linux Docker images which take hours to build. Now this should land without any problem.

[bors]

💔 Test failed - checks-actions

[Manishearth]

@bors retry

network

[bors]

⌛ Testing commit 775eeb3 with merge 9a13ef2...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: Mark-Simulacrum
Pushing 9a13ef2 to master...