openjdk: add 12

* some configure options have been removed upstream

* need a new patch to deal with gcc format warnings

11 remains, as it is an LTS release; all existing users of 11 in
nixpkgs remain on 11 for now.

openjdk/default.nix and openjdk/darwin/default.nix become the
expressions for the current version (12 now; later 13, 14, etc.).

(note: darwin/default.nix was unreferenced; the new version is derived
from darwin/11.nix.)