Fix a bug in Arm64 atomicops. (https://codereview.appspot.com/88800045/)

git-svn-id: http://protobuf.googlecode.com/svn/trunk@524 630680e5-0e50-0410-840e-4b1c322b438d
Down-integrate pending changes from internal repository after previous release.

git-svn-id: http://protobuf.googlecode.com/svn/trunk@525 630680e5-0e50-0410-840e-4b1c322b438d
Fix and include missing tests.
Add missing import of the sys module. Fixes #2.
Don't encode type name in Python 3 since it support unicode. Fixes #3.