Fix micros() for nano without breaking giga

[KurtE]

Fixes #60

The rollover fix for the giga used the 64 bit timer instead of 32 bits. The nano does not have a 64 bit timer, so you need to use the 32 bit instead.

So I know check to see if the processor supports 64 bits if so use it else fall back to 32 bit.

@mjs513 you might want to try this out to see if it fixes the issue for you

[mjs513]

@KurtE
Looks like it fixed the micros issue on the nano:

was hanging with this function:

void clearSerialInput() {
  uint32_t m = micros();
  do {
    if (Serial.read() >= 0) {
      m = micros();
    }
  } while (micros() - m < 10000);
}

no longer hanging

SdFat version: 2.2.3

Assuming the SD is the only SPI device.
Edit DISABLE_CS_PIN to disable an SPI device.

Assuming the SD chip select pin is: 9
Edit SD_CS_PIN to change the SD chip select pin.

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SD initialization failed.
Do not reformat the card!
Is the card correctly inserted?
Is there a wiring/soldering problem?
Is SD_CS_PIN set to the correct value?
Does another SPI device need to be disabled?
SD errorCode: SD_CARD_ERROR_INVALID_CARD_CONFIG = 0x2b
SD errorData = 0x0

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