util/bitscan: add u_foreach_bit macros
this is a standardized (and very slightly improved for usability) version of the macro that has been copied into every vulkan driver includes fixup from Rob Clark <robclark@freedesktop.org> Reviewed-by:Rob Clark <robclark@freedesktop.org> Reviewed-by:
Jason Ekstrand <jason@jlekstrand.net> Part-of: <mesa/mesa!9191> (cherry picked from commit e7c7150d)