[gcc-10-branch] libiberty: Support the new ("v0") mangling scheme in rust-demangle

[Tom Tromey]

Amanieu> rustc can already generate v0 symbols with the
Amanieu> -Zsymbol-mangling-version=v0 flag. We're just going to eventually be
Amanieu> switching this on by default.

Amanieu> I just tested a small program built with that flag and gdb trunk can
Amanieu> successfully demangle the symbols while gdb 10 can't.

Thanks, I ran the gdb test suite using nightly and that flag, and it
worked fine.

Tom