atmel-samd: parallelize lto linking
atmel-samd: parallelize lto linking

This decreases the link time, especially on desktop machines with many CPU
cores.  However, it does come at a slight cost in binary size, making the flash
section about 200 bytes bigger for circuitplayground_express.

Before, linking build-circuitplayground_express/firmware.elf takes
8.8s elapsed time, leaving 3128 bytes free in flash.

After, linking build-circuitplayground_express/firmware.elf takes 2.8s elapsed
time, leaving 2924 bytes free in flash. (-6 seconds, -204 bytes free)

If necessary, we can make this per-board or even per-translation to squeeze full
builds.

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython