fix mpfr test failure on 64 bit Windows

Ref https://github.com/JuliaLang/julia/issues/3399

BigFloat^Int goes to MPFR on every platform other than Win64,
because Win64 is the only place that Julia Int is bigger than Clong.
On Win64 it's going to power_by_squaring resulting in a differently-rounded result.
Merge pull request #7343 from tkelman/patch-4

fix mpfr test failure on 64 bit Windows