[avr] Improved SPI speed on 16bit transfer.

From https://github.com/arduino/Arduino/pull/2376#issuecomment-59671152

Quoting Andrew Kroll:

   [..this commit..] introduces a small delay that can prevent the wait
   loop form iterating when running at the maximum speed. This gives
   you a little more speed, even if it seems counter-intuitive. At
   lower speeds, it is unnoticed. Watch the output on an oscilloscope
   when running full SPI speed, and you should see closer back-to-back
   writes.

Quoting Paul Stoffregen:

   I did quite a bit of experimenting with the NOP addition. The one
   that's in my copy gives about a 10% speedup on AVR.
[avr] Made SPI.begin() and SPI.end() synchronized (Andrew Kroll)
[avr] Added SPI.notUsingInterrupt() (Andrew Kroll)
[avr] Made SPI.usingInterrupt() synchronized (Andrew Kroll)
Fix atomicity issues in SPI::beginTransaction and SPI::endTransaction (Andrew Kroll)

Previously, it could happen that SPI::beginTransaction was
interrupted by an ISR, while it is changing the SPI_AVR_EIMSK
register or interruptSave variable (it seems that there is
a small window after changing SPI_AVR_EIMSK where an interrupt
might still occur). If this happens, interruptSave is overwritten
with an invalid value, permanently disabling the pin interrupts.

To prevent this, disable interrupts globally while changing
these values.