amd64/arm64 pmap: consistently clear PGA_WRITEABLE

We don't consistently clear PGA_WRITEABLE on fictitious, managed pages.
Some functions do, e.g., pmap_remove_all(), but several do not. At
worst, this is just a pessimization, but there is no good reason to be
inconsistent. Clear PGA_WRITEABLE in those that previously did not by
introducing (and using) the helper function pmap_page_is_mapped_locked()
that implements the correct test.

Reviewed by: kib, markj
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D59466