syscall: don't use AF_LINK on hurd

Patch from Svante Signell.

Fixes https://gcc.gnu.org/PR98153

Change-Id: I80a87d882ef7fb786ef121e0a3d84c8541eabb13
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/275939
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>