[vm] Make `Thread::IsAtSafepoint()` only be true if thread is blocked at a "safe" point

During safepoint we can distinguish between

  * owner of the safepoint operation (which is running code)
  * everyone else (which are all blocked

Currently `Thread::IsAtSafepoint()` will return true for both. Since the
thread owning the safepoint operation is running, it's not actually
guaranteed that it's at "safe" point (e.g. to GC or to deopt) - it
really depends on what it's doing.

=> This CL will change it so that only actually parked threads will
have `Thread::IsAtSafepoint()`.

In order to do that we change varrious usages of `IsAtSafepoint()` to be
more precise:

  * `Thread::OwnsSafepoint()`: True if this thread owns the
    active safepoint. The thread is running.

  * `Thread::OwnsGCSafepoint()`: True if the active safepoint is a GC
    (or Deopt) safepoint and this thread owns it. The thread is running.

  * `Thread::OwnsDeoptSafepoint()`: True if the active safepoint is a
    Deopt safepoint and this thread owns it. The thread is running.

  * `Thread::CanAcquireSafepointLocks()`: True if the thread is allowed
    to acquire safepoint locks.

  * `Thread::IsAtSafepoint()`: true if this thread is parked at a
    safepoint

TEST=ci

Change-Id: I1a5a6727e84843ae79e0a344c438da19b7d6d916
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/295781
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>