swr: fix _BitScanForward64 on unix

it must apply to 64 bits types, and use the ctzll intrinsic instead of ctz

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>
15 files changed