Attempt to work around lack of _BitScanReverse64 on WIN32 builds.
Attempt to work around lack of _BitScanReverse64 on WIN32 builds.

(Untested.)

nispaur/pbrt-v3,nispaur/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3,nispaur/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3