Adafruit trinket m0 "bossac not found" on arm64

Trying to add the adafruit samd package complains that bossac is not found. I already have the arduino boards loaded and some of the other adafruit boards but trying to add the adafruit samd boards fails with an error about bossac is not available for your platform.

No source file or test is named in the issue. Start by reproducing installation of the Adafruit SAMD boards package in Arduino IDE 1.6.x on arm64 and capture the bossac lookup failure; done means the package installs without reporting that bossac is unavailable for the platform.

Written by the indexing model from the issue text.