Cross build from aarch64 to x86_64-linux-gnu

[fallenwood]

1. Add cross build from aarch64 to x86_64 linux
2. Add ubuntu-24.04-arm build host

[robbielyman]

i wonder whether it might make sense to move some of the makefile logic into build.zig. similarly, while it's good to verify with CI, the zig cross-compilation story should ideally not actually require us to add different host architectures...