[arch-dev-public] Caution when moving packages built against glibc 2.27

[Evangelos Foutras]

Care should be taken when building against glibc 2.27 (which is currently
in [staging]).

In particular:

   - While glibc 2.27 is in [staging], new rebuilds should not be started.
   - After glibc 2.27 migrates to [testing], packages built against it
   should remain in [testing] until glibc 2.27 lands in [core].

The above considerations must be taken into account because packages built
against glibc 2.27 might depend on versioned symbols that are not found in
glibc 2.26 and thus will not work. [1]

[1] https://bugs.archlinux.org/task/58216