Bug 809697 - [download buttons] Offer linux64 builds to Linux x86_64 user agents.
Bug 527907 - Expose x86_64 Linux builds on the download pages

Added Linux x64 column to all.html with mockup icon, and linux64 builds are now offered on desktop and channel pages download buttons to user agents Linux x86_64.
Added download.linux64 css style.
Merge branch 'bug-809697-expose-linux64'