x86: kgdb support

simplified and streamlined kgdb support on x86, both 32-bit and 64-bit,
based on patch from:

  Subject: kgdb: core-lite
  From: Jason Wessel <jason.wessel@windriver.com>

[ and countless other authors - see the patch for details. ]

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>