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

[Tom Tromey]

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

I checked it in on the gdb 10 branch just now.  I've also filed a bug
noting the backport, per our usual procedure:

https://sourceware.org/bugzilla/show_bug.cgi?id=27194

Tom