libucontext is a lightweight implementation of ucontext for platforms
that do not have a built-in implementation. This allows us to use the
same code to support libucontext as ucontext.

This is a copy of the spice-gtk commit

  commit fa812c88492641005a768c72502a8953bd1223b2
  Author: osy <osy@turing.llc>
  Date:   Fri Mar 4 15:52:48 2022 -0800

    coroutine: add support for libucontext

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>