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
