[M68k] Fix empty builtin va_list kind

Clang _requires_ every target to provide a va_list kind so we shouldn't
put a llvm_unreachable there. Using `VoidPtrBuiltinVaList` because m68k
doesn't have any special ABI for variadic args.

Author: mshockwave

clang/lib/Basic/Targets/M68k.cpp

@@ -159,9 +159,8 @@ const char *M68kTargetInfo::getClobbers() const {
   return "";
 }
 
-M68kTargetInfo::BuiltinVaListKind M68kTargetInfo::getBuiltinVaListKind() const {
-  // FIXME: implement
-  llvm_unreachable("Not implemented yet");
+TargetInfo::BuiltinVaListKind M68kTargetInfo::getBuiltinVaListKind() const {
+  return TargetInfo::VoidPtrBuiltinVaList;
 }
 
 } // namespace targets