pavgb with memory op requires alignment.  This CL disables conversions that use pavgb, and resolves scale by 3/8 unittest for checking alignment works.  The 3/8 code used a pavgb with a memory operand.  tests are added for scaling and allow unaligning on purpose.
BUG=365
TESTED=local change to force unaligned memory fails on some conversions and scaling code.
R=tpsiaki@google.com

Review URL: https://webrtc-codereview.appspot.com/29699004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1114 16f28f9a-4ce2-e073-06de-1de4eb20be90