Update B's size/offset for CvFLAGS to U32, following commit 51c78f1b91bbcd7a.

B.xs contains a table of offsets and sizes for members of the various SV
structures. This needs updating as 51c78f1b91bbcd7a changed CvFLAGS from
U16 to U32. This ommision won't be noticed on little endian platforms, but on
big endian platforms B::CV::CvFLAGS will return 0 instead of the true value.

For: RT #118603