The nrf51x22_256k_16k_s110_8.0.0.ld had a stack size of only 1kB, which
is way too low. Additionally, the indicated _minimum_stack_size (set at
2kB for that chip) isn't respected.
This commit sets the heap end based on the stack size (heap end = RAM
end - stack size) making it much easier to configure.
Additionally, the stack/heap size of nrf52 chips has been set to a more
sane value of 8kB.
4231d4311f