Set kldxref_enable="YES" for ARM images. Without it, the images are missing

Set kldxref_enable="YES" for ARM images. Without it, the images are missing
the /boot/kernel/linker.hints file, which breaks loading some of the modules
with dependencies, eg cfiscsi.ko.

This is a minimal fix for ARM images, in order to safely MFC it before
11.2-RELEASE.  Afterwards, however, I believe we should actually just change
the default (as in, etc/defaults/rc.conf).  The reason is that it's required
for every image that's being cross-built, as kldxref(1) cannot handle files
for non-native architectures.  For the one that is not - amd64 - having it
on by default doesn't change anything - the script is noop if the linker.hints
already exists.

The long-term solution would be to rewrite kldxref(1) to handle other
architectures, and generate linker.hints at build time.

Reviewed by:	gjb@
MFC after:	3 days
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D14534