Revert r333493, which was a temporary fix for 11.2-RELEASE, and instead
switch the default kldxref_enable to YES.

The reason is that it's required for every image that's being cross-built,
as kldxref(8) 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.

MFC after: 2 weeks
Sponsored by: DARPA, AFRL