ar: Avoid overwriting the stdout file stream pointer

This doesn't work with musl, which defines stdout as FILE * const.
Instead, explicitly pass the desired output stream to ar_read_archive().

No functional change intended.

Reviewed by: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34064

(cherry picked from commit 41e6398f9bc1bba4ed872118e742096d692fdfec)