properly support GCC - pass 'I' (int) rather than 'L' (long) to struct.unpack.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: "Yao, Jiewen" <jiewen.yao@intel.com>
Reviewed-by: "Rangarajan, Ravi P" <ravi.p.rangarajan@intel.com>
Reviewed-by: "Ma, Maurice" <maurice.ma@intel.com>
Reviewed-by: "Mudusuru, Giri P" <giri.p.mudusuru@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16474 6f19259b-4bc3-4df7-8a09-765794883524