[NativeAOT] collect garbage when memory is low

[AustinWise]

This matches the existing feature in CoreCLR:

https://github.com/dotnet/runtime/blob/main/src/coreclr/vm/finalizerthread.cpp#L250

I tested with the following program. I observed that the finalizer was not triggered until I created a low memory condition on my computer.

using System;
using System.Runtime.CompilerServices;
using System.Threading;


internal class Program
{
    static void Main(string[] args)
    {
        // The managed finalize loop (which checks for low memory) is not started until the first finalization request.
        // This program does not allocate enough for the GC to request finalization, so we manually trigger it.
        GC.WaitForPendingFinalizers();
        Alloc();
        Thread.Sleep(-1);
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    static void Alloc() => new MyFinalizableObject();
}

class MyFinalizableObject
{
    ~MyFinalizableObject()
    {
        Console.WriteLine("~MyFinalizableObject");
    }
}

Some notes:

• PalCompatibleWaitAny only supports multiple objects on Windows. The low memory event is only available on Windows, so this works out.
• It looks like CoreCLR only collects generation 0 on low memory. I have updated NativeAot to do the same, but I'm not sure that is ideal. Would collecting all generations make more sense?

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[jkotas]

Looks great. Thank you!