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 <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Yonghong Zhu <78d981fe33a016071af5222c56d5cf77f30fb0b2@intel.com>
Reviewed-by: Michael D Kinney <fd20bc543a9f65bb633fac7d08a403c1f1c8eb6c@intel.com>

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