gc_sections needs to be propagated from Core to other modules / executables in CMake configs

There are a few parts to this:

1) It looks like gc_sections is a public feature, and thus consumers of Core need the same flags as set by mkspecs/features/gc_sections.prf. At least that's what I saw when calling qmake -d on lrelease.

2) Currently qt_internal_add_link_flags_gc_sections only sets link flags, whereas it should also set compile flags per specific compiler / target.

This was discovered while investigating lrelease segfaults in cmake static builds at

https://codereview.qt-project.org/c/qt/qttranslations/+/301471

and the fix at

https://codereview.qt-project.org/c/qt/qtbase/+/301837