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.

```
|__/                   |  x86_64-w64-mingw32

julia> BigFloat(10)^1000
9.99999999999999999999999999999999999999999999999999999999999999999999999999968e+999 with
256 bits of precision
```

```
|__/                   |  i686-w64-mingw32

julia> BigFloat(10)^1000
1.000000000000000000000000000000000000000000000000000000000000000000000000000004e+1000 wit
h 256 bits of precision
```

Failure message was:
```
    JULIA test/mpfr
     * mpfr
exception on 1: ERROR: test failed: f > i1
 in error at error.jl:21
 in default_handler at test.jl:19
 in do_test at test.jl:39
while loading mpfr.jl, in expression starting on line 783
ERROR: test failed: f > i1
 in error at error.jl:21
 in default_handler at test.jl:19
 in do_test at test.jl:39
while loading mpfr.jl, in expression starting on line 783
```