Change MPN path to use the architecture-specific assembly folders

This change sets the MPN_PATH parameter of the GMP configure script such that we can take advantage of the optimized assembly present for Atom and ARMv5/ARMv6 rather than relying on GCC's optimizations.

In my application (a simple Mersenne prime finder heavily using mpz_probab_prime_p), I saw a speedup on my Nexus 7 from 6:04 to 3:22 with these changes.