add new stage-qa target: gemdeps

This checks whether rubygem ports have all of their dependencies
in gemspec satisfied by what's currently installed. Sample output:

> Running Q/A tests (stage-qa)

Error: RubyGem dependency archive-tar-minitar = 0.5.2 is not satisfied.

• Error code 1

Stop.
make: stopped in /usr/home/lifanov/src/svn/freebsd/ports/head/archivers/rubygem-fpm

These ports would be broken at runtime.

Big thanks to:

swills - discussion
mat - reviews
antoine - exp runs
sunpoet - fixing several dozens of ports :)

PR: 220605
Reviewed by: mat, sunpoet
Approved by: portmgr (mat)
Differential Revision: https://reviews.freebsd.org/D11841