arm64/riscv pmap: Initialize the pmap's pm_pvchunk field

I believe that there are two reasons that the missing TAILQ
initialization operations haven't caused a problem. First, the TAILQ
head's first field is being initialized to zeroes elsewhere. Second,
the first access to the TAILQ head's last field is by
TAILQ_INSERT_HEAD(), which assigns to the last field without reading
it when the first field is NULL.

Reviewed by: kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D41118