gem: make it possible to generate gems that require rust/cargo

Make sure cargo.mk and gem.mk can work together.
If you want to use rust for your gem, have a look on
devel/rubygem-prometheus-client-mmap as an example.

Differential Revision: https://reviews.freebsd.org/D43292

(cherry picked from commit 87916ecbfac7532b4d3de63efa36ed8f8e637e36)