Documentation: How to use GDB to decode OOPSes

Adds instructions how to use GDB to figure out the exact location of
an OOPS to Documentation/BUG-HUNTING.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>