coroutine-gthread.c: Avoid threading APIs deprecated in GLib 2.31

coroutine-gthread.c: Avoid threading APIs deprecated in GLib 2.31

The GLib threading APIs were revamped in GLib 2.31 and a number
of the old interfaces were deprecated, which means they provoke
compilation warnings (errors if -Werror) now. Add support for the
new interfaces while retaining the old ones so we can still compile
on older versions of GLib too.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>