make 32-bit arch dependencies optional for rpm installers

[dlangBugzillaToGithub]

Martin Nowak (@MartinNowak) reported this on 2015-11-09T21:36:35Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=15308

Description

Newer rpm tools for Fedora [¹] and OpenSuse [²] support weak dependencies (using Recommends: instead of Requires:). Like for the debian package we should use that to support minimal installations.
This requires quite some change for our release scripts as the debian rpmtool used currently doesn't support this.
It's also yet unclear whether such rpm packages are backwards compatible.

[¹]: https://fedoraproject.org/wiki/Packaging:WeakDependencies#Forward_vs_Backward_Dependencies
[²]: https://en.opensuse.org/Libzypp/Dependencies#Weak_dependencies