use archive-product templ. for prod attribute page

#5520

This commit catches requests for 'product attribute' pages
and chooses to use 'archive-product.php' rather than 'archive.php'.

wp-includes/query.php only sets is_post_type_archive when the
query contains the proper post_type argument, which isn't the
case for 'Product Attribute Base' pages. However, it does set
is_archive and is_tax.