Move the implementation of Monitor to managed in CoreCLR

src/coreclr/System.Private.CoreLib/System.Private.CoreLib.csproj

@@ -235,7 +235,9 @@
     <Compile Include="$(BclSourcesRoot)\System\StubHelpers.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Text\StringBuilder.CoreCLR.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Threading\Interlocked.CoreCLR.cs" />
+    <Compile Include="$(BclSourcesRoot)\System\Threading\ManagedThreadId.CoreCLR.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Threading\Monitor.CoreCLR.cs" />
+    <Compile Include="$(BclSourcesRoot)\System\Threading\ObjectHeader.CoreCLR.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Threading\SynchronizationContext.CoreCLR.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Threading\Thread.CoreCLR.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Threading\WaitHandle.CoreCLR.cs" />

src/coreclr/System.Private.CoreLib/src/System/Environment.CoreCLR.cs

@@ -104,7 +104,7 @@ private static unsafe string[] InitializeCommandLineArgs(char* exePath, int argc
         }
 
         [LibraryImport(RuntimeHelpers.QCall, EntryPoint = "Environment_GetProcessorCount")]
-        private static partial int GetProcessorCount();
+        internal static partial int GetProcessorCount();
 
         // Used by VM
         internal static string? GetResourceStringLocal(string key) => SR.GetResourceString(key);

src/coreclr/System.Private.CoreLib/src/System/Threading/ManagedThreadId.CoreCLR.cs

@@ -0,0 +1,17 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Diagnostics;
+
+namespace System.Threading
+{
+    internal static class ManagedThreadId
+    {
+        // This will be initialized by the runtime.
+        [ThreadStatic]
+        private static int t_currentManagedThreadId;
+        internal static int CurrentManagedThreadIdUnchecked => t_currentManagedThreadId;
+
+        public static int Current => t_currentManagedThreadId;
+    }
+}