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:
https://github.com/bbcmicrobit/micropython/blob/a7544718a7138a04168e8e6b283e14e500ffbe8b/inc/microbit/mpconfigport.h#L6