ar: accept but ignore 'T' option

In previous versions of BSD ar -T was an alias for -f -- use only the
first 15 characters of archive member names. In GNU ar and LLVM ar -T
creates a thin archive.

The -f / old BSD ar -T functionality is not particularly useful, and
ignoring -T still results in a usable and compatible (but not thin)
archive.

An exp-run found a few ports invoking ar -T but they all expect thin
archives. In addition, -T will be used to specify thin archives after
a migration to LLVM-ar.

PR: 260523 [exp-run]
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33553