[debugger] Don't list finalizer gc thread if it's not executing managed code (mono...

[debugger] Don't list finalizer gc thread if it's not executing managed code (mono/mono#15618)

* The example that reproduces the bug does this:
var thread = vm.GetThreads()[0];
greetingValue = program.InvokeMethod(thread, greeting, new Value[0]) as StringMirror;

But sometimes the Thread[0] was the MainThread of the program and it works, and other times was the GC Finalizer, but when it tries to run something on GC Finalizer thread, the thread is not suspended and throws an exception.
In the fix I removed the Finalizer thread if it's not executing a managed code of the list that is returned when CMD_VM_ALL_THREADS is called.

Fix mono/mono#13311

Commit migrated from https://github.com/mono/mono/commit/73128453daff8966f1c8eeacb734579971b02218