Rewrite/remove GPL-only code

[XVilka]

Currently, src/cxx/ and src/gnu_v2 contain the GPL-only code copied from libiberty library (part of GCC).

To allow linking the non-GPL or LGPL software we should rewrite it in LGPL or another compatible license.

[wargio]

the GPL code can be skipped on compilation time: https://github.com/rizinorg/rz-libdemangle/blob/main/meson_options.txt#L1

[ret2libc]

I believe this can be closed for now as having a build time option is already good IMHO. If somebody wants this for commercial reasons they can probably pay someone to write this.

[wargio]

i would keep this instead. no need to close it

[wargio]

C++ demangler could be rewritten using https://github.com/ianlancetaylor/demangle/blob/master/demangle.go as base.