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

See discussion here: dlang/druntime#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.

abi.dd

@@ -410,6 +410,7 @@ $(I Type):
     $(I TypeNull)
     $(I TypeTuple)
     $(I TypeVector)
+    $(I Internal)
 
 $(I Shared):
     $(B O) $(I Type)
@@ -591,6 +592,9 @@ $(I TypeNull):
 
 $(I TypeTuple):
     $(B B) $(I Number) $(I Arguments)
+
+$(I Internal):
+    $(B Z)
 )
 )