Throw InvalidOperationException on RuntimeMethodHandle.GetFunctionPointer() for generic methods

[steveharter]

Fixes #101664

On CoreClr, the previous exception was InvalidProgramException. On Mono, the call caused a crash.

[lambdageek]

@steveharter what should this do:

public class G<T>
{
    public static T Method(T x) => x;
}

public class Program
{
    public static void Main()
    {
        var mi = typeof(G<>).GetMethod("Method", BindingFlags.Public|BindingFlags.Static);
        var h = mi.MethodHandle;
        var fp = h.GetFunctionPointer();
        Console.WriteLine(fp);
    }
}  

On CoreCLR this works (returns some kind of non-null value for fp).
On Mono interp it works too.
On Mono JIT it crashes:

* Assertion at /Users/runner/work/1/s/src/mono/mono/mini/mini-generic-sharing.c:3368, condition `!mono_class_is_gtd (class_vtable->klass)' not met

(same behavior on .NET 9 preview 6 and .NET 8, across both runtimes)

[lambdageek]

if we want to also rule out methods from a generic type definition, then you may want to add || mono_class_is_gtd (method->klass)

[steveharter]

Yes we should throw if the method is not callable meaning a "true" generic method and a generic method that uses the type parameters from the owning class. It seems like IsGenericMethod() should return true even if the type parameters come from the owning class, but that's a different problem...

I'll make another pass here. Thanks

[buyaa-n]

Merging as @steveharter is out