Remove sad.asm from the processing lib, move satd_sad from the encoder to the common lib

sad.asm as used in processing is an exact subset of the
code in satd_sad.asm in the encoder.
Remove an unused assembly source file

Nothing within processing uses functions from this file.
Merge pull request #252 from mstorsjo/share-processing-asm

Merge identical assembly code between the processing and encoder libs