[sgen] Always report object moves on the GC thread.

[sgen] Always report object moves on the GC thread.

This avoids exposing the SGen worker threads through the profiler APIs. Doing
so had two main problems:

* It's easy for a user of the profiler API to accidentally mess with the state
  of the worker thread in all sorts of ways, which could break the GC.
* It's been a long-standing guarantee that any profiler API callback will be
  invoked on a thread that is, at the very least, attached to the runtime via
  the mono-threads infrastructure. This is not the case for SGen workers.

This fixes a deadlock in the log profiler when using major=marksweep-conc.