create USES=gem and update rubygem- ports to use it
Note that for now ports still have to have USE_RUBY=yes to use USES=gem
PR: 209041
Differential Revision: https://reviews.freebsd.org/D6070
Set NO_ARCH for remaining rubygem-* ports with no arch dependent files
Obtained from: DEVELOPER_MODE
With Hat: ruby@
Sponsored by: RideCharge Inc / Curb