ar: diff reduction against ELF Tool Chain

Remove ar_mode_* wrappers and call ar_read_archive / ar_write_archive
directly.

(This was originally committed in d20e9e02db3d but reverted due to a
regression, now fixed with an update from cy@.)

Obtained from: ELF Tool Chain
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31496