Changing stack is not usually instrumentation friendly.
Allows to enable the usage of Valgrind memcheck calling some
valgrind macros.

This is a copy of the spice-gtk commit:

  commit 942bd130ddd3c331045c1d3e4f580909901d285a
  Author: Frediano Ziglio <freddy77@gmail.com>
  Date:   Thu Aug 13 14:31:52 2020 +0100

      Allows to use Valgrind with ucontext coroutines

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