Fix test to unconditionally create a GNU-format archive.
Fix test to unconditionally create a GNU-format archive.

On Darwin targets, llvm-ar creates a Darwin format archive by default,
which ld.lld can't read, so it was printing an unexpected error.



git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@355894 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld