Fix #85: Let each x86 configs set optimization level
Add `COPT` variable in `Config.x86_common` to set the optimization level (defaulting to `-O2`). Then each `Config.x86` file can set `COPT` as desired if the default doesn't work. Thus, `Config.x86_linux` sets `COPT` to `-O1`, but others can use the default value. See issue #68.