x86/traps: Clean up error exception handler definitions

So I was reading the exception handler generation code and got a real
headache looking at the unstructured mess that our DO_ERROR*()
generation code is today.

Make it more readable.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: http://lkml.kernel.org/n/tip-kuabysiykvUJpgus35lhnhvs@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>