bitops: fix rol/ror when shift is zero

bitops: fix rol/ror when shift is zero

All the variants for rol/ror have a bug in case where the shift == 0.
For example rol32, would generate:

    return (word << 0) | (word >> 32);

Which though works, would be flagged as a runtime error on clang's
sanitizer.

Suggested-by: Richard Henderson <[email protected]>
Signed-off-by: Nikunj A Dadhania <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Signed-off-by: David Gibson <[email protected]>