x86: more AVX2 framework, AVX2 functions, plus some existing asm tweaks

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

AVX2 functions:
mc_chroma
intra_sad_x3_16x16
last64
ads
hpel
dct4
idct4
quant_4x4x4
quant_4x4
quant_4x4_dc
quant_8x8
SAD_X3/X4
SATD
var
SSD
zigzag interleave
weightp
weightb
intra_sad_8x8_x9
decimate
integral
x86: SSSE3 ads_mvs

~55% faster ads in benchasm, ~15-30% in real encoding.
~4% faster "placebo" preset overall.