Avoid UB from misaligned loads in variance_sse2.c
Avoid UB from misaligned loads in variance_sse2.c

The undefined behaviour came from READ64, whose loads compile to a
MOVD but which is technically incorrect if p is misaligned. This patch
rewrites it, and the other loads and stores in the file, to use the
xx_* functions from synonyms.h

BUG=aomedia:912

Change-Id: Ic2fae623ef3b609dacd0a830a7cc63653291202f

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom