HLE_OS: Implement %n in GetStringVA

%n writes to a pointer that's provided as a parameter.
We didn't have a custom implementation of this before,
meaning that %n would trigger a write to the host
memory instead of the emulated memory!

Source/Core/Core/HLE/HLE_OS.cpp

@@ -137,6 +137,11 @@ std::string GetStringVA(u32 strReg)
         result += StringFromFormat("%x", (u32)Parameter);
         break;
 
+      case 'n':
+        PowerPC::HostWrite_U32(static_cast<u32>(result.size()), static_cast<u32>(Parameter));
+        // %n doesn't output anything, so the result variable is untouched
+        break;
+
       default:
         result += StringFromFormat(ArgumentBuffer.c_str(), Parameter);
         break;