add option -fmetrowerks-abi (unimplemented)

gcc/gcc/c-family/c.opt

@@ -1254,6 +1254,9 @@ fabi-compat-version=
 C++ ObjC++ Joined RejectNegative UInteger Var(flag_abi_compat_version) Init(-1)
 The version of the C++ ABI used for -Wabi warnings and link compatibility aliases.
 
+fmetrowerks-abi
+C++ ObjC++ Var(flag_metrowerks_abi)
+
 faccess-control
 C++ ObjC++ Var(flag_access_control) Init(1)
 Enforce class member access control semantics.

gcc/gcc/cp/mangle.c

@@ -785,7 +785,8 @@ write_mangled_name (const tree decl, bool top_level)
     }
   else
     {
-      write_string ("_Z");
+      if (!flag_metrowerks_abi)
+	write_string ("_Z");
       write_encoding (decl);
     }
 }