FWIW, this ABI break was reverted 2 weeks after the LWN article was released:

  commit 2f438e20ab591641760e97458d5d1569942eced5
  Author: Stefan Liebler <___.ibm.com>
  Date:   Thu Jul 31 20:04:54 2014 +0200
  
      S/390: Revert the jmp_buf/ucontext_t ABI change.