Use 32-bit GMP size limit on unknown architectures
Use 32-bit GMP size limit on unknown architectures

There doesn't seem to be a general-purpose way to determine architecture
word sizes from CPP as invoked by GHC, so we now fall back on the 32-bit
GMP size limit for unknown architectures. This is always safe, and can
be amended to allow larger words on other architectures as needed.

Closes #542.

GaloisInc/cryptol,GaloisInc/cryptol,GaloisInc/cryptol