[Mono-list] Mono-generator .cctor() has incorrect sig (#45204)

[Piers Haken]

Monodis shows the following signature for mono-generated .cctors:

    .method compilercontrolled static  specialname  rtspecialname
           default void .cctor()  cil managed

Whereas .NET-generated .cctors have the following sig:

    .method private static  hidebysig  specialname  rtspecialname
           default void .cctor()  cil managed

Piers.