runtime: add page tracer

This change adds a new GODEBUG flag called pagetrace that writes a
low-overhead trace of how pages of memory are managed by the Go runtime.

The page tracer is kept behind a GOEXPERIMENT flag due to a potential
security risk for setuid binaries.

Change-Id: I6f4a2447d02693c25214400846a5d2832ad6e5c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/444157
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/syscall_unix_test.go

@@ -0,0 +1,25 @@
+// Copyright 2022 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 runtime_test
+
+import (
+	"runtime"
+	"syscall"
+	"testing"
+)
+
+func TestSyscallFlagAlignment(t *testing.T) {
+	// TODO(mknyszek): Check other flags.
+	check := func(name string, got, want int) {
+		if got != want {
+			t.Errorf("flag %s does not line up: got %d, want %d", name, got, want)
+		}
+	}
+	check("O_WRONLY", runtime.O_WRONLY, syscall.O_WRONLY)
+	check("O_CREAT", runtime.O_CREAT, syscall.O_CREAT)
+	check("O_TRUNC", runtime.O_TRUNC, syscall.O_TRUNC)
+}