fix build on 32-bit systems

Addresses https://github.com/facebook/libphenom/issues/13
The underlying issue is that 32-bit systems don't have 64-bit CAS
primitives so we need to size our ring bits to the system pointer
size instead.

I did some light testing of this by amending tpool.t to use 64 threads
and it seemed to work (the concern would be whether we overflow
MAX_RINGS)