[Coverage] Map regions from system headers (#76950)

In 21551951, the
"system-headers-coverage" option has been added but not used in all
necessary places.

This is the recommit since it has been reverted in
faef68bc



Potential reviewers: @gulfemsavrun @petrhosek

Co-authored-by: Manuel Kalettka <manuel.kalettka@kernkonzept.com>