Cherry pick r516 from upstream protobuf

Fix compilation for arm64.

TBR=cpu@chromium.org
R=mark@chromium.org
BUG=314720

Review URL: https://codereview.chromium.org/163723003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251345 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251345