Compress Linux debug info packages with dwz

In QTBUG-3934 we've got debug_info packages released for Linux. However, it's easy to find out that these debug_info packages are a few times larger, than corresponding MSVC debug_info packages, even if we look only at sizes of 7z archives.

It's possible to improve this situation by using dwz tool (https://fedoraproject.org/wiki/Features/DwarfCompressor) which reduces size of DWARF files without altering their meaning. Applying dwz substantially reduces not only size of uncompressed files, but size of 7z archive as well

This commit adds dwz to provisioning of RHEL 7.6 where our Linux binaries are built, and also contains numbers of size savings which I've obtained for QtWebKit: https://codereview.qt-project.org/c/qt/qt5/+/292717

I can imagine 2 ways of implementing this: add support directly to build system, i.e. new configure option or feature and underlying qmake code, or run it separately on build artifacts.