I’m reporting the problem and (hopefully) the solution, but I think we’d
better double-check this.

The problem: Running the test below in a loop sometimes gets a SIGSEGV
in the child process (on x86_64, libc 2.22.)

--8<---------------cut here---------------start------------->8---
(use-modules (guix build syscalls) (ice-9 match))

(match (clone (logior CLONE_NEWUSER
                      CLONE_CHILD_SETTID
                      CLONE_CHILD_CLEARTID
                      SIGCHLD))
  (0
   (throw 'x))                                    ;XXX: sometimes segfaults
  (pid
   (match (waitpid pid)
     ((_ . status)
      (pk 'status status)
      (exit (not (status:term-sig status)))))))
--8<---------------cut here---------------end--------------->8---

Looking at (guix build syscalls) though, I see an ABI mismatch between
our definition and the actual ‘syscall’ C function, and between our
‘clone’ definition and the actual C function.

This leads to the attached patch, which also fixes the above problem for me.