Torbutton should pass TBB's OS and architecture to www.tpo download pages

Torbutton should pass TBB's operating system and architecture (not the OS and architecture that it's running on; e.g. TBB for 32-bit Linux run on FreeBSD-AMD64 should pass “Linux” and “i686”) to JavaScript running on www.tpo/download/download-easy and www.tpo/download/download. This should probably be done by adding a ‘fragment identifier’ (e.g. ...#foozer) to the URL when browsing to those URLs without a fragment identifier.

Ideally, the fragment identifier would still point users to the correct section of the page even when JavaScript is disabled.