On MSVC, casting i32 to i64 then multiplying gets compiled into a call
to __allmul, which is slow. Use the __emul intrinsic instead.
GCC 4.6+ appears optimise this case automatically and doesn't need any
special handling.
This reduces the cost of ComputeShortPath by about 50% (testing AI vs AI
on Oasis 01).
This was SVN commit r13873.