re PR go/88060 (../../../gcc-8.2.0/libgo/go/syscall/libcall_linux_utimesnano.go:17:18: error: reference to undefined name ‘_AT_FDCWD’)

	PR go/88060
    syscall: always define _AT_FDCWD and IPv6MTUInfo
    
    They aren't defined by old versions of glibc, but are required by the
    code in syscall_linux.go.
    
    Reviewed-on: https://go-review.googlesource.com/c/150697

From-SVN: r266333