swr: missing _BitScanForward64 on 32 bits win

the code does not compile on 32 bits systems
for mingw we can use gcc intrinsics like the unix side
for msvc a generic implementation is provided

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Krzysztof Raszkowski <krzysztof.raszkowski@intel.com>
Reviewed-by: Jan Zielinski <jan.zielinski@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6705>
1 file changed