backports of 32ef9e5054ec ("Makefile.debug: re-enable debug info for .S files")

Dear stable kernel maintainers, Our production kernel team and ChromeOS kernel teams are reporting that they are unable to symbolize addresses of symbols defined in assembly sources due to a regression I caused with commit a66049e2cf0e ("Kbuild: make DWARF version a choice") I fixed this upstream with commit 32ef9e5054ec ("Makefile.debug: re-enable debug info for .S files") but I think this is infeasible to backport through to 4.19.y.

Do the attached branch-specific variants look acceptable?