Hello,

This adds "tramp-frame.[hc]" as a generic signal trampoline frame 
unwinder.  The client architectures, per the revised change to 
ppcnbsd-tdep.c (it superseeds my original), just needs to provide the 
signal trampoline instruction sequence and a method to initialize the 
trad_frame_cache.

Thoughts, I'll look to commit the tramp-frame parts in a few days.

committed to my tramp branch,
Andrew