cmd/pprof: install as go tool pprof

LGTM=bradfitz, r
R=r, bradfitz
CC=golang-codereviews
https://golang.org/cl/168320043

src/cmd/go/pkg.go

@@ -399,6 +399,7 @@ var goTools = map[string]targetDir{
 	"cmd/nm":                               toTool,
 	"cmd/objdump":                          toTool,
 	"cmd/pack":                             toTool,
+	"cmd/pprof":                            toTool,
 	"cmd/yacc":                             toTool,
 	"golang.org/x/tools/cmd/cover":         toTool,
 	"golang.org/x/tools/cmd/godoc":         toBin,

src/cmd/pprof/README

+The pprof in this directory is adapted from the pprof used inside Google
+for C++, Java, and Go programs. Because it was developed for that broader
+context, it is overgeneralized when used here for the specific use case
+of profiling standard Go programs. However, we've left the abstractions
+intact in order to share updates between this copy and Google's internal one.
+
+Please do not take the level of abstraction in this program as an example
+to follow in your own.

src/cmd/pprof/doc.go

+// Copyright 2014 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.
+
+// Pprof interprets and displays profiles of Go programs.
+//
+// Usage:
+//
+//	go tool pprof binary profile
+//
+// For more information, see http://blog.golang.org/profiling-go-programs.
+package main