Fix pmap_is_prefaultable() on arm64 and riscv

The current implementations never correctly return TRUE. In all cases,
when they currently return TRUE, they should have returned FALSE. And,
in some cases, when they currently return FALSE, they should have
returned TRUE. Except for its effects on performance, specifically,
additional page faults and pointless calls to pmap_enter_quick() that
abort, this error is harmless. That is why it has gone unnoticed.

Add a comment to the amd64, arm64, and riscv implementations
describing how their return values are computed.

Reviewed by: kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D33659