MdeModulePkg: PcdSetNxForStack: enable dynamism
    
    Allow platforms to instantiate this PCD as PcdsDynamic and PcdsDynamicEx
    too, not just PcdsFixedAtBuild and PcdsPatchableInModule.
    
    Cc: Star Zeng <star.zeng@intel.com>
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>
    Reviewed-by: Star Zeng <star.zeng@intel.com>
    Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
    
    git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18468 6f19259b-4bc3-4df7-8a09-765794883524