loader.efi: add "gop blt <on|off>" command

Some systems have very slow console output and it may be about either
wrong memory attributes are set or gop->Blt() implementation is bad.

We do not have good way to set memory attributes, but we can
choose which Blt() to use (or we can set "gop off" to fall back on
use of SimpleTextOutput protocol).

This update adds argument for "gop" command to switch gop->Blt() use.
Note, this update does not fix the problem, but allows us to try to
understand the possible cause.

PR: 254381
Reported by: Michael Galassi

Reviewed by: manu, imp
Differential Revision: https://reviews.freebsd.org/D49073