Fixed cortex-M hooks for RTOS

[cmaglie]

Added RTOS handler like in the SAM core.

Fixes #4

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b17_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request Fixed cortex-M hooks for RTOS #13
5. Select one of the boards under SAMD Pull Request Fixed cortex-M hooks for RTOS #13 in Tools->Board menu
6. Compile/Upload as usual

[cmaglie]

See also: http://forum.arduino.cc/index.php?topic=332080.0