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
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