MdeModulePkg: PcdSetNxForStack: enable dynamism

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