samd/mcu/samd21: Enable MICROPY_STACK_CHECK on SAMD21.

Increases firmware size by +140 bytes and uses +4 extra bytes of RAM, but
allows the test suite to run without crashing.

Signed-off-by: Damien George <damien@micropython.org>

ports/samd/mcu/samd21/mpconfigmcu.h

@@ -16,6 +16,7 @@
 
 // Selected extensions beyond the basic features set.
 #define MICROPY_ENABLE_FINALISER        (1)
+#define MICROPY_STACK_CHECK             (1)
 #define MICROPY_KBD_EXCEPTION           (1)
 #define MICROPY_HELPER_REPL             (1)
 #define MICROPY_REPL_AUTO_INDENT        (1)