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

[Amanieu d'Antras via Gcc-patches]

Could this commit be backported to the gcc-10 branch? We would like to
switch the default name mangling scheme in Rust but are currently
blocked on upstream tool support.

commit 84096498a7bd788599d4a7ca63543fc7c297645e
    libiberty: Support the new ("v0") mangling scheme in rust-demangle