zh_CN/gccgo.po: extract gccgo messages into a separate file

Github can't display diff for file as large as gcc.po.

Signed-off-by: minux <minux.ma@gmail.com>
README: update notes about zh_CN/gccgo.po.

Signed-off-by: minux <minux.ma@gmail.com>
zh_CN/gccgo.po: replay changes in rev 411648af.

Signed-off-by: minux <minux.ma@gmail.com>