fix type mismatch between mp_limb_t and uint64_t on aarch64

[mars-risc0]

Fix for issue #351 by declaring vars in fr.hpp and fr.cpp as mp_limb_t instead of uint64_t, since uint64_t is defined as unsigned long long on aarch64, not unsigned long as on other platforms, whereas mp_limb_t is always defined as unsigned long.