target: xscale more human vector catching

Replace hexadecimal masks for vector catching with words
representing the caught exception, such as dabt for data
abort, etc ...
This way, the new xscale command is :
 - xscale vector_catch
   Reads back to the user the current vector catching status
 - xscale vector_catch reset dabt pabt
   Sets the caught vectors to data abort and prefetch abort
   for example.

This is mostly taken from Cortex-M3 openocd code.

Change-Id: I66591d5796f0e07f0f31edc8d28722e1e48aa8c5
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Reviewed-on: http://openocd.zylin.com/1456
Tested-by: jenkins
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>