BaseTools: Create and use a shared value for 'MSFT' from DataType

I see lots of 'MSFT' throughout code and this can reduce them.

Cc: Bob Feng <Bob.c.Feng@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>

BaseTools/Source/Python/Common/DataType.py

@@ -554,3 +554,5 @@ PACK_CODE_BY_SIZE = {8:'=Q',
                      1:'=B',
                      0:'=B',
                     16:""}
+
+TAB_COMPILER_MSFT = 'MSFT'