Use archive_format for determining if gnu style archives should be emitted
Use archive_format for determining if gnu style archives should be emitted

Fixes #893

graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust