[Singular/f19] (4 commits) ...new normalize in f21+ only

[Rex Dieter]

Summary of changes:

  de8f909... ExclusiveArch: %ix86 x86_64 (*)
  cce1b6b... Update normaliz interface for normaliz 2.8 and later. (*)
  97624f2... ntl6 now in f19 too (*)
  19a904e... new normalize in f21+ only (*)

(*) This commit already existed in another branch; no separate mail sent