archivers/rpm4: Replace calls to xargs by gxargs

Latest version of archivers/rpm4 uses GNU specific option
when calls xargs. This causes the proccess to fail.

This commit adds misc/findutils as a run dependency and
replaces xargs calls by its GNU variant gxargs.

Also Bump portrevision.

PR: 244606
Reported by: Brian Zou <zoujiaqing@gmail.com>