Documentation: openrisc: Update toolchain binaries URL

The old development toolchain binaries were hosted in the or1k-gcc
development github repo release page.  However, now that we have all
code upstream I cut releases from stable upstream tarballs.  It does not
make sense to tag the or1k-gcc github repo releases for these stable
releases.

Update the toolchain binaries URL to point to where they are now hosted
on the or1k-toolchain-build github release page.

Signed-off-by: Stafford Horne <shorne@gmail.com>