feature request: set step-mode smart

[Ilya Basin]

Hi! Sometimes you need to step into a function that is called via a
jump table. In this case gdb behaves inconveniently:
Either you leave default step-mode and gdb can't step into that
function or you set step-mode on and gdb will step into all functions
without sources.

I think, gdb should try to stepi a fixed number of instructions (~100)
searching for a code line and if not found, step out.