Fix building with MinGW x64

[esaulenka]

It's a minor fix to compile tests on mingw compiler from MSys2.

For some reason actual mingw version doesn't include __WORDSIZE definition with your common includes #include <inttypes.h> #include <stddef.h>
But it's included somewhere in Arduino.cpp (haven't checked, is it stdio.h or time.h).
In that case BigNumberUtil.cpp compiled with BIGNUMBER_LIMB_32BIT, but all .ino tests compiled with BIGNUMBER_LIMB_64BIT.

Fixed code works well both with mingw and original linux gcc (I cannot check Mac version).