Add build tag to omit grpc

This is needed so we can build the runc shim without grpc as a
transative dependency.
With this change the runc shim binary went from 14MB to 11MB.
The RSS from an idle shim went from about 17MB to 14MB (back around
where it was in in 1.7).

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

cmd/go-buildtag/main.go

@@ -0,0 +1,96 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io"
+	"os"
+)
+
+func main() {
+	wrFl := flag.Bool("w", false, "Write to file")
+	tagsFl := flag.String("tags", "", "Tags to add")
+
+	flag.Parse()
+
+	if flag.NArg() < 1 {
+		fmt.Fprintln(os.Stderr, "Usage: go-buildtag <options> <file1> [<file2> <file3> ...]")
+		os.Exit(1)
+	}
+
+	wr := *wrFl
+	tags := *tagsFl
+	for i := 0; i < flag.NArg(); i++ {
+		p := flag.Arg(i)
+		if err := handle(wr, tags, p); err != nil {
+			fmt.Fprintln(os.Stderr, p+":", err)
+			os.Exit(2)
+		}
+	}
+}
+
+func handle(wr bool, tags, p string) (retErr error) {
+	out := os.Stdout
+
+	rdr, err := os.Open(p)
+	if err != nil {
+		return fmt.Errorf("could not open source file: %w", err)
+	}
+	defer rdr.Close()
+
+	if wr {
+		f, err := os.Create(p + ".tmp")
+		if err != nil {
+			return fmt.Errorf("error creatign temp file")
+		}
+		defer func() {
+			if retErr != nil {
+				os.Remove(f.Name())
+			}
+		}()
+
+		defer f.Close()
+		out = f
+	}
+
+	_, err = fmt.Fprintf(out, "//go:build %s\n\n", tags)
+	if err != nil {
+		return fmt.Errorf("error writing build tags: %w", err)
+	}
+
+	_, err = io.Copy(out, rdr)
+	if err != nil {
+		return fmt.Errorf("error copying original content to temp file")
+	}
+
+	rdr.Close()
+	if wr {
+		out.Close()
+
+		if err := os.Remove(p); err != nil {
+			return fmt.Errorf("error removing original source file: %w", err)
+		}
+
+		if err := os.Rename(p+".tmp", p); err != nil {
+			return fmt.Errorf("error replacing original source: %w", err)
+		}
+	}
+
+	return nil
+}