After digging into amd64 pmap details I came to the conclusion
that it isn't helpful to merge them into one.
They are so different. We have a different page table architecture,
and we have a completely different address space layout.
There might be common algorithms, but this the same as with
any other architecture using in-memory page-tables.
Hiding the differences in macros or turning the code into
an #ifdef mess would hinder further maintenance.
(If you want a bad example, look at the mc68020/040 stuff.)

best regards
Matthias