Merge branch 'staging'

Main change: glibc: 2.25-x -> 2.26-y, containing security fixes,
and various features and deprecations.  Unfortunately, some of the
latter still cause (transitively) a couple hundred newly failing jobs.
I'm not delaying anymore, so that we have the security fix on master.
I mainly patched gcc, llvm and icu, but I can't fix everything...