Silence idiotic warning on two's complement of an unsigned

MSVC gives:

warning C4146: unary minus operator applied to unsigned type,
               result still unsigned

When finds -b where b is an unsigned integer. So rewrite the two's
complement in a way to avoid the warning. Theoretically the new
version is slower, but in practice changes nothing.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>