kldxref: Reduce divergence between per-architecture files

Note that relbase is always 0 for DSOs so its omission for __KLD_SHARED
architectures was not a bug in practice.

Whilst here, also parenthesise the dest offset for where to avoid
transiently creating an out-of-bounds pointer, which is UB (though even
on CHERI architectures, where capability bounds compression can result
in that creating invalid capabilities that will trap on dereference,
optimisation will reassociate to the correct form in practice and thus
work just fine).