internal/poll, internal/syscall/unix, net: move and export fcntl syscall wrapper

This will allow to use the fcntl syscall in packages other than
internal/poll.

For #60181

Change-Id: I76703766a655f2343c61dad95faf81aad58e007f
Reviewed-on: https://go-review.googlesource.com/c/go/+/494916
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/internal/syscall/unix/fcntl_unix.go

@@ -0,0 +1,25 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package unix
+
+import (
+	"syscall"
+	_ "unsafe" // for go:linkname
+)
+
+// Implemented in the runtime package.
+//
+//go:linkname fcntl runtime.fcntl
+func fcntl(fd int32, cmd int32, arg int32) int32
+
+func Fcntl(fd int, cmd int, arg int) (int, error) {
+	val := fcntl(int32(fd), int32(cmd), int32(arg))
+	if val < 0 {
+		return 0, syscall.Errno(-val)
+	}
+	return int(val), nil
+}