powerpc: Allow the max stack trace depth to be configured

On my screen, when something crashes, I only have space for maybe 16
functions of the stack trace before the information above it scrolls
off the screen.  It's easy to hack the kernel to print out only that
much, but it's harder to remember to do it.  This introduces a config
option for it so that I can keep the setting in my config.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>