amd64: Zero-fill AP PCPU pages

At least KMSAN relies on zero-initialization of AP PCPU regions, see
commit 4b136ef259ce.

Prior to commit af1c6d3f3013 these were allocated with allocpages() in
the amd64 pmap, which always returns zero-initialized memory.

Reviewed by: kib
Fixes: af1c6d3f3013 ("amd64: do not leak pcpu pages")
MFC after: 3 days
Sponsored by: Klara, Inc.
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D42241