nrf/mpconfigport: Reduce GC stack size for nrf51.
This frees 128 bytes of .bss RAM on the nRF51, at the cost of possibly
more expensive GC cycles. Leave it as-is on the nRF52 as that chip has a
lot more RAM.
This is also done in the micro:bit:
#else
#define MICROPY_FLOAT_IMPL (MICROPY_FLOAT_IMPL_FLOAT)
#endif
+#if NRF51
+#define MICROPY_ALLOC_GC_STACK_SIZE (32)
+#endif
#define MICROPY_OPT_COMPUTED_GOTO (0)
#define MICROPY_OPT_CACHE_MAP_LOOKUP_IN_BYTECODE (0)
