Problem resolved! As usually solution is quite simple: use GPIO interrupt but test which button has been pressed. GPIO iterrupt:
void buttonInt(void){ // disable button interrupt
M8C_DisableIntMask(INT_MSK0, INT_MSK0_GPIO);
if (Right_Data_ADDR & Right_MASK) buttonRightPressed = 1;
if (Left_Data_ADDR & Left_MASK) buttonLeftPressed = 1;
if (Select_Data_ADDR & Select_MASK) buttonSelectPressed = 1;
}