This patch fixes a bug I introduced in commit 
d53c26c753a39b80a338fb85bd41f75a49374842, when I was refactoring code to 
prepare for the addition of new Nios II ISA variants.  One of the new 
variants we were testing did not have an appropriate "trap" encoding 
usable for all breakpoints, so we had to tweak GDB to use "break 31" 
instead of "trap 31".  While that works fine for bare-metal, it's 
incompatible with the published Nios II ABI documentation for Linux 
systems, which explicitly requires "trap 31".  Moreover, using "break 
31" causes a kernel hang.  Oops!  So, this patch puts it back to using 
"trap 31", the way it was before.  We've discussed this with Altera and 
they are going to fix the new ISA instead of changing the ABI.

OK to commit?

-Sandra