x86/misc: early cleanup

x86/misc: early cleanup

Various bits of cleanup without functional impact as far as the series goes,
but make subsequent patches cleaner.

* WARN_ON(1) is just WARN().
* Replace hand-crafted rolled stack printing with fatal_trap().
* 16 BSS bytes is overkill for an empty idtr to triple fault with.  Construct
  it on the stack using an appropriate struct, and correct the asm memory
  constraint.
* Fix watchdog asymmetry in panic().  machine_halt() needs just as much
  watchdog care as machine_restart(), but it should be up to the arch
  implementation of machine_{halt,restart}() to play with the watchdog.
* unsigned and const correctness for trapstr(), along with whitespace cleanup.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>