amd64: initialize APs kpmap_store in init_secondary()

The APs pcpu area is zeroed in init_secondary() by pcpu_init(), so the
early initialization in pmap_bootstrap() is nop.

Fixes: 42f722e721cd010ae5759a4b0d3b7b93c2b9cad2ESC
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D39945