Ignore ZPP in gcovr (GH-21623)

The ZPP macros materialize into optimized code with many error branches that
aren't useful to test for every function. Ignore these lines completely by using
the gcovr --exclude-lines-by-pattern flag. For codecov to pick up on this
change, we generate the xml file manually and point to it in the codecov action.
Also move the ignore paths from codecov.yml to Makefile.gcov.