Test on ARM64

[0xEAB]

LDC should work on ARM64, as well.
This should be tested.

Manually, at least; still better than nothing.

[jnms-me]

Building and testing other platforms can easily be done with docker buildx, which uses QEMU when necessary.
E.g. Debian supports these platforms:

• linux/386
• linux/amd64/v1,v2,v3
• linux/arm/v5,v7
• linux/arm64/v8
• linux/mips64le
• linux/ppc64le
• linux/s390x (IBM Z)