I have never seen win-x86_64 builds of XULRunner 6. But it is possible that they are existing, since FireFox nighly builds have win-x86_64 version now http://nightly.mozilla.org/.
The problem is that the lastest version of XULRunner supported by Eclipse is 1.9.x. So even if we find these XULRunner 6 builds, it unlikely helps us.