Don't include a 64 when building 64bit architectures on flatpak

Summary:
It's not a thing on flatpak and it makes packaging weird.
Should fix https://github.com/flatpak/freedesktop-sdk-base/issues/22

Reviewers: Frameworks, cgiboudeaux

Reviewed By: cgiboudeaux

Subscribers: cgiboudeaux, eszlari, kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D13102