[reflection] Initialize default ALC gchandle in GetLoadContext (#60399)

If we try to get the ALC of an assembly from the default context before the
managed default ALC object has been created, the native gchandle has a null
target.
Ensure it is not null by explicitly referencing the Default ALC.

Fixes #60348

Co-authored-by: Aleksey Kliger <alklig@microsoft.com>

Author: github-actions[bot]

src/mono/System.Private.CoreLib/src/System/Runtime/Loader/AssemblyLoadContext.Mono.cs

@@ -80,6 +80,7 @@ internal static Assembly[] GetLoadedAssemblies()
             // We only support looking up load context for runtime assemblies.
             if (rtAsm != null)
             {
+                var _ = Default;  // ensure the default ALC is initialized.
                 RuntimeAssembly runtimeAssembly = rtAsm;
                 IntPtr ptrAssemblyLoadContext = GetLoadContextForAssembly(runtimeAssembly);
                 loadContextForAssembly = GetAssemblyLoadContext(ptrAssemblyLoadContext);