Support demangling __Z (two underscores) automatically

[xPaw]

There's this code:

https://github.com/ianlancetaylor/demangle/blob/81f5be970ecae4046912a026004100b0535be16e/demangle.go#L153-L156

I don't see why it can't support double underscores by itself either. Currently it requires manually stripping the underscore, but then it breaks demangling strings that already have only one underscore.

For example: echo "__Znwm" | c++filt -_