Older compiler versions warned about unknown attributes with
-Wattributes instead of -Wunknown-attributes. Use both and suppress
warnings about unknown warnings options too.

Change-Id: I22252426e73d4984471b9b8415479bd7b57ba0c5
Reviewed-on: https://swiftshader-review.googlesource.com/11289Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>