Also, get rid of the boilerplate code using some macros. STACK_V(h,f) means
"call f twice, stacking vertically at an offset of h". STACK_H(w,f)
means "call f twice, stacking horizontally at an offset of w".

Note that functions like subtract_128x64 are now only defined when the
equivalent block sizes (e.g. BLOCK_128x64) are defined. As such, we
have to fix up subtract_test.cc so it doesn't try to call
aom_highbd_subtract_block_sse2 with unsupported sizes.

BUG=aomedia:684

Change-Id: I5b0fefe70e4083786d11d25cdd5dcf02823bae7b