Enhance GDB's abillity WRT stack traces where signal handlers are involved.

Enhance GDB's abillity WRT stack traces where signal handlers are involved.
Correct backtrace was made more complex when the new signal trampoline
was introduced to support more than 32 signals, while keeping a modified
version of the old signal trampoline.

The 'where' command will now show:

	#2  <signal handler called>

where appropiate.

Submitted by:	Tor.Egge@fast.no