delayMicroseconds() timing is off for 1 MHz clock.

[damellis]

We should probably add a 1 MHz case to the delayMicroseconds() function because that speed is relatively common with ATtiny's and other standalone microcontrollers. For one report, see: damellis/attiny#12

[microtherion]

Also discussed in issue arduino/Arduino#256

[cmaglie]

Fixed with: #1678