layers: MR121, Use _BitScanForward for u_ffs on windows
layers: MR121, Use _BitScanForward for u_ffs on windows

The linux ffs function scans from least-significant-bit.
To match that, u_ffs on windows should use _BitScanForward instead of
_BitScanReverse.

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools