x64: nested exceptions

The scenario

  run elf file
    demand fault
      allocation
        leak detector tracking
          backtrace_safe()
            page fault

leads to a nested exception.  Add support for it by allocating an extra
stack.

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