Prepare a rootfs for ARM64 system. chroot into it and emulate every binary with qemu-user. Now replace gcc etc with one configured as cross-compiler, but still point its sysroot to the usual path (/usr/...).

This is equivalent to building a x86-to-aarch64 cross-compiler and point the sysroot to a ARM64 rootfs, but more intuitive to users unfamiliar with cross toolchains, and more friendly to those mad configure scripts which insist on assuming host = target and detect everything automatically.

You forgot the following steps: Profile to see what's the slowest thing Make a native version of that. In FreeBSD package builder setup, we also have /bin/sh and a few other heavily used binaries that are native because it shaves hours or days off a package run (itself takes 10 days, even optimized like this).