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.

Fixes https://gcc.gnu.org/PR88060

Change-Id: Id894b306b7cf7c6e352a068ae0856b638799e049
Reviewed-on: https://go-review.googlesource.com/c/150697
Reviewed-by: Cherry Zhang <cherryyz@google.com>