Sparsify the vm_page_dump bitmap

Sparsify the vm_page_dump bitmap

On Ampere Altra systems, the sparse population of RAM within the
physical address space causes the vm_page_dump bitmap to be much
larger than necessary, increasing the size from ~8 Mib to > 2 Gib
(and overflowing `int` for the size).

Changing the page dump bitmap also changes the minidump file
format, so changes are also necessary in libkvm.

Reviewed by:	jhb
Approved by:	scottl (implicit)
MFC after:	1 week
Sponsored by:	Ampere Computing, Inc.
Differential Revision:	https://reviews.freebsd.org/D26131