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

[Tom Tromey]

Amanieu> Demangler support for v0 has already been upstreamed in GCC's libiberty and is
Amanieu> available in gdb-binutils master. However we would like the demangler to be
Amanieu> backported to GDB 10 so that debugging experience does not regress when we
Amanieu> switch to generating v0 symbol names by default.

I don't know whether we plan to do another release from the GDB 10
branch.  Maybe Joel could say.

Also, is there a way to test that this works correctly?  I'd imagine
there just has to be a flag day sometime and then we find out?  Or has
someone built Rust and tried it with GDB trunk?

Anyway, it's fine by me to do this.  If there's no objections I can do
it sometime.  You may want to ping after a week or so in case I forget.

thanks,
Tom