add alpine-aarch64 continuous Integration builds

[calvin2021y]

There is no alpine-aarch64 ldc in latest successful Continuous Integration builds.

static alpine-aarch64 can work for any version of linux, verfy useful for try it on any version of Linux distribution.

there already alpine-x86_64, can we add alpine-aarch64 ?

[kinke]

Yeah this should be pretty straight-forward - using a native Linux AArch64 CI runner and an according Alpine container image.

[kinke]

Oh well, the GitHub runner lacks proper support for it: actions/runner#801

[calvin2021y]

I know nothing about GitHub Actions. If it's possible to download the Alpine root filesystem, extract it, and then execute a chroot, maybe it could work without container.

[kinke]

There's a wild workaround in that issue which seems to (still) work in #4943.