Fix link against runtime.goyield.

This function does not exist in Go 1.13. We need to add an adaptor
to build against Go 1.13, which is the default Ubuntu version.

PiperOrigin-RevId: 343929132

pkg/sync/BUILD

@@ -40,6 +40,7 @@ go_library(
         "race_unsafe.go",
         "rwmutex_unsafe.go",
         "seqcount.go",
+        "spin_legacy_unsafe.go",
         "spin_unsafe.go",
         "sync.go",
     ],

pkg/sync/spin_legacy_unsafe.go

@@ -0,0 +1,25 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build go1.13
+// +build !go1.14
+
+package sync
+
+import (
+	"runtime"
+	_ "unsafe" // for go:linkname
+)
+
+//go:linkname canSpin sync.runtime_canSpin
+func canSpin(i int) bool
+
+//go:linkname doSpin sync.runtime_doSpin
+func doSpin()
+
+func goyield() {
+	// goyield is not available until Go 1.14.
+	runtime.Gosched()
+}

pkg/sync/spin_unsafe.go

@@ -3,7 +3,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build go1.13
+// +build go1.14
 // +build !go1.17
 
 // Check go:linkname function signatures when updating Go version.