faster decoding speed with Visual

faster decoding speed with Visual

by enabling the fast decoder path.
Visual requires a different set of macro constants to detect x86 / x64.

On my laptop, decoding speed on x64 went up from 3.12 to 3.45 GB/s.
32-bit is less impressive, though still favorable,
with speed increasing from 2.55 to 2.60 GB/s.

So both cases are now enabled.

Suggested by Bartosz Taudul (@wolfpld).