Revised lib/Integers.v to make it parametric w.r.t. word size.

Introduced Int.iwordsize and used it in place of "Int.repr 32" or
"Int.repr (Z_of_nat wordsize)".



git-svn-id: https://yquem.inria.fr/compcert/svn/compcert/trunk@1182 fca1b0fc-160b-0410-b1d3-a4f43f01ea2e