os: mingw version of Readdir() and Stat() implemented

src/pkg/os/Makefile

src/pkg/os/dir_mingw.go

src/pkg/os/file.go

src/pkg/os/file_mingw.go

src/pkg/os/file_unix.go

src/pkg/os/stat_mingw.go

src/pkg/syscall/syscall_mingw.go

src/pkg/syscall/syscall_mingw_386.go

src/pkg/syscall/zerrors_mingw_386.go

src/pkg/syscall/zsyscall_mingw_386.go

src/pkg/syscall/ztypes_mingw_386.go