`$toplevel/system` of a system closure with `x86_64` kernel and `i686` userland should contain "x86_64-linux".
If `$toplevel/system` contains "i686-linux", the closure will be run using `qemu-system-i386`, which is able to run `x86_64` kernel on most Intel CPU, but fails on AMD.
So this fix is for a rare case of `x86_64` kernel + `i686` userland + AMD CPU