x86: correctly check stack alignment for Atom hadamard_ac

Regression in r2265 (only affected compilers with broken stack alignment,
like ICL on win32).
weightp: improve scale/offset search, chroma

Rescale the scale factor if the offset clips. This makes weightp more effective
in fades to/from white (and an other situation that requires big offsets).

Search more than 1 scale factor and more than 1 offset, depending on --subme.

Try to find the optimal chroma denominator instead of hardcoding it.

Overall improvement: a few percent in fade-heavy clips, such as a sample from
Avatar: TLA.
AVX2 dct4/idct4/quant_4x4x4/SAD_X3/X4/SATD/var/SSD