Add to your services field in operating-system:

            (service qemu-binfmt-service-type
             (qemu-binfmt-configuration
              (platforms (lookup-qemu-platforms "arm" "aarch64" "ppc" "mips64el"))
              (guix-support? #t)))

and reconfigure.

Then

$ guix environment -s armhf-linux linux-libre
(env)$ make oldconfig
(env)$ make nconfig