Fix incomplete stack traces by gdb.

Fix incomplete stack traces by gdb.

Gdb tries unwinding a stack frame by analyzing the function prologue.
If it can not find the beginning of the function, which happens for
stripped binaries, etc., it will resort to guessing.  It then assumes
that the function is a frame-less function without any local stack
variables.  This of course is wrong for almost all functions.

We work around this problem by assuming a valid stack frame.

Previous versions of gdb were broken the same way, but libbfd would
actually compensate with another bug which would simply report a wrong
function start address -- the address of a preceding, known function.
Because most functions indeed use a proper stack frame, this would
trick gdb into doing the unwinding properly.