Add arm64 code for downsample.
Add ARM64 Code and UnitTest for reconstruction
Merge pull request #1092 from dongzha/AddRecARM64CodeAndUT

Add ARM64 Code and UnitTest for reconstruction
Merge pull request #1091 from zhilwang/arm64-downsample

Add arm64 code for downsample.
Merge remote-tracking branch 'upstream/master' into travis.osx