Fix CancelFutureTest for single-uring and libc++

Florian Fischer (4):
      [SubmitActor] don't manually invalidate broken chains after linux 5.15
      [IoContext] do not use Future after allowing its destruction
      [Semaphore] make default constructor more explicit
      Merge branch 'fix-cancel-on-single-uring' into fix-ci

 emper/Semaphore.hpp      |  2 +-
 emper/io/IoContext.cpp   | 10 +++++-----
 emper/io/SubmitActor.cpp |  3 ++-
 3 files changed, 8 insertions(+), 7 deletions(-)