doom: fix to work on FPGA, which doesn't have hardware timer
doom: fix to work on FPGA, which doesn't have hardware timer

This is also smoother in the emulator

jbush001/NyuziProcessor,FulcronZ/NyuziProcessor,FulcronZ/NyuziProcessor,jbush001/NyuziProcessor,FulcronZ/NyuziProcessor,hoangt/NyuziProcessor,jbush001/NyuziProcessor,hoangt/NyuziProcessor,hoangt/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor,hoangt/NyuziProcessor,FulcronZ/NyuziProcessor,jbush001/NyuziProcessor