discovery: add GNU backtrace support

GNU backtrace is much better for automatic adaption. The downside
is that it possibly crashes with SIGSEGV. With 3 as max. backtrace
depth it seems to be stable, now. It finds _Znwm, _Znwj and malloc
calls this way.
ugtrain,libmemhack: add GNU backtrace support

If GNU backtrace() doesn't crash during discovery stage 3, then
we can use it in libmemhack as well. We introduce the 'use_gbt'
config command to activate it. All configured stack offsets are
ignored then.