Update exec.c (copyied from efi/loader/arch/arm64/exec.c) to allow
execution of aarch64 kernels. This includes a new trampile code that
handles copying the UEFI memory map, if available from the Linux FDT
provided PA. This is a complete implementation now, able to boot from
the LinuxBoot environment on an aarch64 server that onyl offers
LinuxBoot (though a workaround for the gitv3 inability to re-init is not
upstream in FreeBSD yet).
Sponsored by: Netflix