Add the Z 'type' to the mangling ABI to denote internal symbols.

See discussion here: https://github.com/D-Programming-Language/druntime/pull/725

D already creates mangled names using the Z symbol for internal symbols, e.g.
_D3foo3Bar6__vtblZ for the virtual table for class foo.Bar. The ABI makes no
mention of these symbols. This pull request adds Z to the mangling ABI to make
these official symbols. The linked pull request implements that demangling.