releng-gce: Advertise the availability of UEFI support in GCE images.

The amd64 and arm64 images supported UEFI, mark it as so users can take
advantage of UEFI boot on GCE. This is already done on FreeBSD
14.0-RELEASE but never codified into the release tools (and should).

PR: conf/276532
Reviewed by: lwhsu
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D43557