Build linux/arm64 docker images

[dtretyakov]

As of now mstorsjo/llvm-mingw docker image is build for linux/amd64 platform.

It would be great to support at least linux/arm64 target, since the base ubuntu image supports the following:

• linux/amd64
• linux/arm/v7
• linux/arm64/v8
• linux/ppc64le
• linux/s390x

Reference:

• Multi-platform image with GitHub Actions

[mstorsjo]

The latest prerelease, with LLVM 18.1.0 RC 3, is out now, and now there's an aarch64 version of the docker image as well.

I had to do a minor last minute tweak to get this built correctly, see 908f88f7e5c06228b130a7ab2be7cde77ad45e4f. I had set load: true for the docker/build-push-action job, to make the new image visible in docker images after building (to see what it looks like when doing a test build) when using buildx. However when setting push: true for a real release, that broke: https://github.com/mstorsjo/llvm-mingw/actions/runs/7989160838/job/21815208766