loader.efi(8): the amd64 loader doesn't do protected mode

The amd64 UEFI loader executes in long mode, not protected mode.

Reviewed by: kib
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D57568

(cherry picked from commit f771deb193d5743e74515614b7cec40d68ab02ca)