x64: Enable sleeping in fault context

In preparation for enabling demand paging, enable sleeping in fault
context by using a per-thread exception stack for normal faults and
per-CPU exception stack for nested faults.

Avi Kivity explains:

  Before [demand paging] can even hope to work, we need to enable
  sleeping in fault context.  Right now each cpu has its own exception
  stack, which leads immediately to stack corruption:

  thread 1 faults
  enters exception stack
  tries to take mutex
  scheduler switches to thread 2
  thread 2 faults
  enters same exception stack

  So we need to switch stacks.  This can be done in the same way as for
  interrupt stacks (see thread::switch_to()).

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>