fix BigInt --> Int64 conversion on 32-bit platforms [fix #5315]

These definitions get replaced on 64-bit platforms since Clong ===
Int64 and Culong === Uint64, so they only apply on 32-bit platforms
where pulling out the hi and lo words is necessary.
Merge pull request #5316 from JuliaLang/sk/big2int64

fix BigInt --> Int64 conversion on 32-bit platforms [fix #5315]