Have preload_delete_name() free pages backing preloaded data.

On i386 and amd64, add a vm_phys segment for physical memory used to
store the kernel binary and other preloaded data. This makes it
possible to free such memory back to the system once it is no longer
needed, e.g., when a preloaded kernel module is unloaded. Previously,
it would have remained unused.

Reviewed by: kib, royger
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D16330