1. make the program go buildable
2. update os.EINVAL and runtime.Cgocalls()
3. wrap mpz_div_2exp() and mpz_mul_2exp to support both
   pre-5.0 and post-5.0 gmp (we really have no reason to
   restrict ourselves to gmp 5.0+)

R=golang-dev, remyoudompheng, iant
CC=golang-dev
https://golang.org/cl/5847061