Treat the 32 bit stride registers as signed.

Alternatively, we could make the stride arguments ptrdiff_t instead
of int, and changing all of the assembly to operate on these
registers with their full 64 bit width, but that's a much larger
and more intrusive change (and risks missing some operation, which
would clamp the intermediates to 32 bit still).

Fixes: https://trac.ffmpeg.org/ticket/9985Signed-off-by: Martin Storsjö <martin@martin.st>