Always use 64-bit physical addresses for dump_avail[] in minidumps

As of r365978, minidumps include a copy of dump_avail[]. This is an
array of vm_paddr_t ranges. libkvm walks the array assuming that
sizeof(vm_paddr_t) is equal to the platform "word size", but that's not
correct on some platforms. For instance, i386 uses a 64-bit vm_paddr_t.

Fix the problem by always dumping 64-bit addresses. On platforms where
vm_paddr_t is 32 bits wide, namely arm and mips (sometimes), translate
dump_avail[] to an array of uint64_t ranges. With this change, libkvm
no longer needs to maintain a notion of the target word size, so get rid
of it.

This is a no-op on platforms where sizeof(vm_paddr_t) == 8.

Reviewed by: alc, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27082