Adding atomic ops for MIPS64.

This change adds atomic operations for MIPS64.

BUG=400684

Review URL: https://codereview.chromium.org/450343002

git-svn-id: http://src.chromium.org/svn/trunk/src/base@289624 4ff67af0-8c30-449e-8e8b-ad334ec8d88c