ar: add -U (unique) option to disable -D (deterministic) mode

This is required in order for us to support deterministic mode by
default. If multiple -D or -U options are specified on the command
line, the final one takes precedence. GNU ar also uses -U for this.

An equivalent change will be applied to ELF Tool Chain's version of ar.

PR: 196929
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3175