Allow usage of rust for rubygems

[mfechner]

Summary

Make it possible to use rust/cargo together with rubygems.
The commits are also hosted here:
https://gitlab.fechner.net/mfechner/Gitlab/-/commits/rust-prometheus-client-mmap

The patch is designed in a way that it should not have any impact to any existing rubygens port.

The port that requires cargo support from ports need to set some additional configurations, see also commit message included in patch file

Test Plan

I tested all ports used by gitlab-ce to be buildable.

[arrowd]

Mk/Uses/gem.mk

86

This is not needed anymore, I guess? Uses/cabal.mk appends to _USES_<target> unconditionally.

[mfechner]

Mk/Uses/gem.mk

86

I have added it to not change the logic. Not sure if any rubygem port overwrite the do-extract target.
At least the command does not find anything:

rg  -g '**/rubygem**/Makefile' do-extract

I prefer to keep it, as it enables the maintainer to overwrite it, if necessary.

[mfechner]

I removed now the if condition around the do-extract target, it is not used.