BaseTools: change the Division Operator in the expression

BaseTools: change the Division Operator in the expression

PEP 238 -- Changing the Division Operator
x/y to return a reasonable approximation of the mathematical result
    of the division ("true division")
x//y to return the floor ("floor division")

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Yunhua Feng <yunhuax.feng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>