replaced 64-bit multiplication in difficulty.cpp with a portable version
replaced 64-bit multiplication in difficulty.cpp with a portable version

eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero,eiabea/bitmonero,eiabea/bitmonero,ranok/bitmonero,eiabea/bitmonero,ranok/bitmonero,eiabea/bitmonero,eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero