This patch updates the nios2 prologue analyzer to handle some recent-ish 
GCC code generation changes:

* In GCC r223083, the instruction emitted when a stack overflow is 
detected and in other trap situations was changed from "break" to 
"trap".  This patch tweaks the prologue analyzer to recognize both the 
old and new stack checking sequences.

* In GCC r225787, the stack overflow checking logic was changed so that 
now there is only a single check being emitted instead of one after each 
stack adjustment.  This doesn't require code changes to GDB, but I 
tidied up the comments that explain what can appear in the prologue.

* It seemed to me to be an oversight that "trap" and "break" 
instructions not related to stack overflow checking don't indicate that 
we've advanced past the end of the prologue, as do control transfer 
instructions.  GCC only generates these from calls to __builtin_trap() 
or in situations where it has detected some other trap situation with 
undefined behavior, like a null pointer dereference, which shouldn't 
appear in prologue code.

OK to commit?

-Sandra