pmap: standardize promotion conditions between amd64 and arm64

On amd64, don't abort promotion due to a missing accessed bit in a
mapping before possibly write protecting that mapping. Previously,
in some cases, we might not repromote after madvise(MADV_FREE) because
there was no write fault to trigger the repromotion. Conversely, on
arm64, don't pointlessly, yet harmlessly, write protect physical pages
that aren't part of the physical superpage.

Don't count aborted promotions due to explicit promotion prohibition
(arm64) or hardware errata (amd64) as ordinary promotion failures.

Reviewed by: kib, markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D36916