Fix array overreads that caused miscompilation in gcc 4.8
x86inc: create xm# and ym#, analagous to m#

For when we want to mix simd sizes within one function.
x86: more AVX2 framework, AVX2 quant_8x8

Use 32-byte alignment instead of 16 to satisfy AVX2 using new macros.
Change the alignment of malloc.
avx2 idct 16x16

I don't plan to actually push this (I'll probably convert it to 8x8 because
we almost never use 16x16).
I also need to benchmark this (it's not certain to be faster), so I need
real hardware.