ar: diff reduction against ELF Tool Chain

• Drop exit status from bsdar_errc. ELF Tool Chain always returns EXIT_FAILURE in bsdar_errc.

• Remove ar_mode_* wrappers and call ar_read_archive / ar_write_archive directly.

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