archivers/gtar: switch the default archive to stdin/stdout

The gtar port tries to use the same default archive location as the
base system tar. Starting with libarchive 3.6.0, bsdtar defaults
to reading/writing the archive from stdin/to stdout in the common
case where the /dev/sa0 device does not exist on a system.

Using "-" as the default archive file is also the GNU upstream
default.

PR: 271260
Reported by: Michael Stone <mstone2001@msn.com>