net: link with networking libraries when net package is in use

Fixes #10221.

Change-Id: Ib23805494d8af1946360bfea767f9727e2504dc5
Reviewed-on: https://go-review.googlesource.com/7941Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

src/net/sendfile_solaris.go

@@ -10,6 +10,10 @@ import (
 	"syscall"
 )
 
+// Not strictly needed, but very helpful for debugging, see issue #10221.
+//go:cgo_import_dynamic _ _ "libsendfile.so"
+//go:cgo_import_dynamic _ _ "libsocket.so"
+
 // maxSendfileSize is the largest chunk size we ask the kernel to copy
 // at a time.
 const maxSendfileSize int = 4 << 20