runtime/cgo: on Solaris link against -lxnet

Fixes #32205

Change-Id: If5b1d32a5e19ff5d9337862c07fb77890d19d69f
Reviewed-on: https://go-review.googlesource.com/c/go/+/183379
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

src/runtime/cgo/cgo.go

@@ -21,6 +21,7 @@ package cgo
 #cgo netbsd LDFLAGS: -lpthread
 #cgo openbsd LDFLAGS: -lpthread
 #cgo aix LDFLAGS: -Wl,-berok
+#cgo solaris LDFLAGS: -lxnet
 
 #cgo CFLAGS: -Wall -Werror