Add cross-compilation support to Makefile

[nohajc]

At the moment, I'm porting init.krun to FreeBSD (hoping to open that PR soon) and I thought it would be neat to have the option to compile on macOS without spinning up a VM if one so chooses.

This relies on clang and lld (the LLVM linker). Plus, you need to provide path to a suitable sysroot. I tested the build by copying /usr/include, /usr/lib/aarch64-linux-gnu and /usr/lib/gcc/aarch64-linux-gnu from a Debian Bookworm system.

Just wasn't sure if I should strip the binary or not. Right now it's configured to only strip debug_info.

[nohajc]

I suppose I should also document this in README if the change is accepted.

[nohajc]

This can return arm64 on macOS or aarch64 on Linux but clang seems happy with either in the target triple.

[slp]

Having to gather all binaries for putting together a sysroot is a bit cumbersome. I was wondering if we could, perhaps, provide a Containerfile that would build init/init on macOS Podman/Docker. I think that would simplify a lot the process for the users.

[nohajc]

My idea was to minimize build dependencies (i.e. it should be doable without any container runtime / virtualization solution). I first added this for FreeBSD on my branch where making a sysroot was as simple as downloading and unpacking base.txz.

Maybe I could write a similar script for Linux. For example, it's possible to use skopeo and umoci to download and extract files from any OCI image. But I'm sure one could find Debian tarballs or something like that as well.

[nohajc]

Yep, you can just download these four deb packages, unpack with ar and tar and you have a working 40MB sysroot:

https://deb.debian.org/debian/pool/main/g/glibc/libc6_2.42-2_arm64.deb
https://deb.debian.org/debian/pool/main/g/glibc/libc6-dev_2.42-2_arm64.deb
https://deb.debian.org/debian/pool/main/g/gcc-12/libgcc-12-dev_12.5.0-6_arm64.deb
https://deb.debian.org/debian/pool/main/l/linux/linux-libc-dev_6.12.57-1_all.deb

[nohajc]

Ok, I updated the Makefile to generate a Debian sysroot automatically on macOS.
It downloads https://deb.debian.org/debian/dists/$(DEBIAN_DIST)/main/binary-$(ARCH)/Packages.xz and parses it to find the following packages:

libc6, libc6-dev, libgcc-12-dev, linux-libc-dev

These are downloaded and unpacked to linux-sysroot/. The only homebrew dependencies are lld and xz.

This should ultimately simplify homebrew packaging of libkrun as well.