MdePkg/IndustryStandard: Add SMBIOS anchor string & length defines.

MdePkg/IndustryStandard: Add SMBIOS anchor string & length defines.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3805

Add SMBIOS spec defined anchor strings & length defines.

 - '_SM_' chapter 5.2.1 SMBIOS 2.1 (32-bit) Entry Point
 - '_SM3_' chapter 5.2.2 SMBIOS 3.0 (64-bit) Entry Point

Changes from v1 to v2:
 - Replace non-ASCII characters

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Cc: Andrew Fish <afish@apple.com>
Signed-off-by: Giri Mudusuru <girim@apple.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>