MdePkg/Library/BaseCpuLibNull: Add StandardSignatureIsAuthenticAMD()

CpuLib.h exposes StandardSignatureIsAuthenticAMD() API and we require
stub function in its BaseCpuLibNull library instance to avoid potential
link issue.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Signed-off-by: Qing Huang <qing.huang@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>