Infinite loop in sigstep.exp

[Andrew Cagney]

> Somehow, stepi off the "ret" instruction just goes back to the
> beginning of the function instead of stepping into "<signal handler
> called">.
> 
> This blows up my test bed, so I have to fix it.
> 
> My first plan is to add some counters to prevent infinite loops
> with "exp_continue".  I'm open to other ideas.

A [k]failing pattern for:

	"Breakpoint .*$gdb_prompt "

would also help.

Andrew