cmake: use project dir instead of cmake dir when building libunwind

The EE CMakeConfig.txt adds CE as a sub-directory. For this to work,
we should use project dir instead of cmake dir in CE cmake files,
see commit d8097325 ("cmake: align folders dependencies").

Fixes commit 14f93aee ("libunwind: improve incremental
build/rebuild").

Follow-up #8019

NO_DOC=build fix
NO_TEST=build fix
NO_CHANGELOG=build fix