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.
x86: more AVX2 framework, AVX2 functions

Use 32-byte alignment instead of 16 to satisfy AVX2 using new macros.
Change the alignment of malloc.

AVX2 functions:
mc_chroma
nal_escape
intra_sad_x3_16x16
last64
ads
hpel
dct4
idct4
quant_4x4x4
quant_4x4
quant_4x4_dc
quant_8x8
SAD_X3/X4
SATD
var
SSD