[libc++] Migrate Linaro Buildkite builders to Github Actions

I would like to reduce libc++'s reliance on Buildkite for pre-merge checks since it creates problems for other projects in the monorepo. Specifically, they see libc++'s Buildkite job on their PR status checks even though it ends up being a no-op. I tried other ways to trigger our Buildkite pipeline that would allow using Github's builtin file filtering instead (see [this PR](https://github.com/llvm/llvm-project/pull/165928)), but I don't think it's actually possible to do that while retaining a good UI in PRs.

Hence, I think we want to move as many of our jobs as possible to Github Actions instead, and perhaps remove our Buildkite integration entirely. What's required here would be for Linaro to switch from installing the Buildkite agent on their machines to the ARM Linux Github self-hosted runner instead:

```
# For ARM
curl -o actions-runner-linux-arm-2.329.0.tar.gz -L https://github.com/actions/runner/releases/download/v2.329.0/actions-runner-linux-arm-2.329.0.tar.gz

# For ARM64
curl -o actions-runner-linux-arm64-2.329.0.tar.gz -L https://github.com/actions/runner/releases/download/v2.329.0/actions-runner-linux-arm64-2.329.0.tar.gz
```

Then, run this on the runners:

```
./config.sh --url https://github.com/llvm/llvm-project --token SOME_TOKEN
```

`SOME_TOKEN` will have to be provided by someone who has access to this on https://github.com/llvm/llvm-project (@tstellar ?)