libkvm - Add ability to access userspace from kgdb on cores

libkvm - Add ability to access userspace from kgdb on cores

* Use %cr3 from the dumppcb instead of KPML4phys on x86_64, and similarly
  for i386, to access the full page table as of when the panic occured
  instead of just the kernel page table.

* minidumps do not dump userspace so userspace will still not be available,
  but this gives us the option of sysctl'ing off minidumps when userspace
  access is desired, and kgdb will then be able to access the current
  userspace context as of the panic, as well.