Experiment with QEMU testing in FreeBSD CI once 13.x goes out of support #25390
Description
Conveniently, this port exists: https://cgit.freebsd.org/ports/tree/devel/freebsd-sysroot
Inconveniently, it's currently tracking FreeBSD 13.x which we don't support when cross-compiling.
This is a temporary state of affairs, however; FreeBSD 14.x was when FreeBSD started being cross-compilable from Linux, making it actually practical for me to add support for FreeBSD libc cross-compilation in Zig. But that also meant that I didn't add support for 13.x despite that release still being supported upstream. But going forward, we will of course support every release of FreeBSD after 14.x.
So, once 13.x goes out of support (when 15.x releases later this year), the above port will switch over to tracking 14.x. That means we can then install it for all supported targets on the FreeBSD CI machine and point QEMU to it. It looks like user-mode emulation on FreeBSD currently only supports aarch64, arm, and riscv64 - meaning no powerpc64 and powerpc64le - but that's still a lot better than only covering x86_64.