buildman: Update link to the most recent prebuilt ARC toolachin

To troubleshoot unexpected bhavior during building and what's more
important during execution it is strongly recommended to use recent
ARC toolchain, and so we're now referring to arc-2016.09 which is the
latest as of today.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Reviewed-by: Simon Glass <sjg@chromium.org>