delayMicroseconds(): added support for 1Mhz, 12Mhz and 24Mhz

1Mhz is a default clock speed on Atmega328, many users run stand alone microcontroller on the
internal 1Mhz clock to save battery power. Up until now delayMicroseconds() function wasn't taking this frequencies into an account defaulting to 8Mhz resulting incorrect timing. Patch includes support for 12 and 24 Mhz clock as well