loader: Do not force comconsole for arm and arm64

This makes GOP not probed on some situation (AMD Card on PCIe slot
with EDK2 as we have a SERIAL_IO_PROTOCOL compatible uart).

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D32992
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 123b5b8763778e83b6816ad9db62a9b956055c32)