In my personal opinion, we should not need to support unaligned YUY2
pixel maps. They should always be aligned to at least 32 bits, and the
current code assumes just 16 bits. However checkasm does test for
unaligned input bitmaps. QEMU accepts it, but real hardware dose not.

In this particular case, we can at the same time improve performance and
handle unaligned inputs, so do just that.

uyvytoyuv422_c:      104379.0
uyvytoyuv422_c:      104060.0
uyvytoyuv422_rvv_i32: 25284.0 (before)
uyvytoyuv422_rvv_i32: 19303.2 (after)