Using the SAMD21 SWCLK (pin45) as digital output has a bug, it requires a patch, see here:
http://www.avdweb.nl/arduino/samd21/zero.html#h12-3-swclk
const int SWCLK = 32;
pinMode(SWCLK, INPUT); // patch to use the SWCLK pin as output
pinMode(SWCLK, OUTPUT);
digitalWrite(SWCLK, 0);