Mk/Uses: make it possible to compile gems that need rust to build

This commits defines same variables in a way that they can be overwritten
by a port.
Refactored the extract target to work together with cargo.mk.

The gem.mk file will require more work to be easily usable for gems that
required rust. Most of the work is now outsourced to the port.

I added comments to parts that needs more love.

To test these modification, the following commands can be used:
(make sure you adjust the path to match your environment)
rm /usr/local/poudriere/ports/gitlab/distfiles/rubygem/gapic-common-*.gem
rm /usr/local/poudriere/ports/gitlab/distfiles/rubygem/prome*.gem
rm /usr/local/poudriere/ports/gitlab/distfiles/rust/crates/ahash-*crate
/usr/local/poudriere/ports/gitlab/distfiles/*.gem
cd /usr/local/poudriere/ports/gitlab/devel/rubygem-prometheus-client-mmap
make makesum
cd /usr/local/poudriere/ports/gitlab/devel/rubygem-gapic-common
make makesum
poudriere testport -p gitlab -j 132amd64 devel/rubygem-gapic-common && poudriere testport -p gitlab -j 132amd64 devel/rubygem-prometheus-client-mmap