BaseTools: add /Gw to CC_FLAGS for VS2013 and higher tool chain tags

BaseTools: add /Gw to CC_FLAGS for VS2013 and higher tool chain tags

The /Gw flag does a better job at size optimization than use of the
GLOBAL_REMOVE_IF_UNREFERENCED macro that is currently used for VS20xx
tool chains to remove unreferenced global variables.
This patch add /Gw to CC_FLAGS for VS2013 and higher tool chain tags.

Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>