minidump: De-duplicate is_dumpable()

The function is identical in each minidump implementation, so move it to
vm_phys.c. The only slight exception is powerpc where the function was
public, for use in moea64_scan_pmap().

Reviewed by: kib, markj, imp (earlier version)
MFC after: 2 weeks
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D31884