Move CommunicationBuffer from stack to global variable.

Move CommunicationBuffer from stack to global variable.

We had put communication buffer to Runtime memory,
so that SMI handler can know it is not used by OS.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: "Yao, Jiewen" <jiewen.yao@intel.com>
Reviewed-by: "Zeng, Star" <star.zeng@intel.com>
Reviewed-by: "Kinney, Michael D" <michael.d.kinney@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18949 6f19259b-4bc3-4df7-8a09-765794883524