m68k/mac: Improve NMI handler

mac_nmi_handler() is useless in its present form and locks up my PowerBook
180. Let's throw out the dead code and make it do something useful: print
a register dump and a stack trace.

mac_debug_handler() is also dead code. Remove it along with its static
data.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>