Summary:
There's a race condition like this:

  # A foreign pointer gets promoted to the last generation
  # It has its finalizer called manually
  # We start shutting down the runtime in `hs_exit_` from the main
    thread
  # A minor GC starts running (`scheduleDoGC`) on one of the threads
  # The minor GC notices that we're in `SCHED_INTERRUPTING` state and
    advances to `SCHED_SHUTTING_DOWN`
  # The main thread tries to do major GC (with `scheduleDoGC`), but it
    exits early because we're in `SCHED_SHUTTING_DOWN` state
  # We end up with a `DEAD_WEAK` left on the list of weak pointers of
    the last generation, because it relied on major GC removing it from
    that list

This change:
  * Ignores DEAD_WEAK finalizers when shutting down
  * Makes the major GC on shutdown more likely
  * Fixes a bogus assert

Test Plan:
before this diff https://ghc.haskell.org/trac/ghc/ticket/7170#comment:5
reproduced and after it doesn't

Reviewers: ezyang, austin, simonmar

Reviewed By: simonmar

Subscribers: bgamari, thomie

Differential Revision: https://phabricator.haskell.org/D921

GHC Trac Issues: #7170