cmd/addr2line: exit 0 for --help

This is what pprof expects, or else it won't use the program.
And if it doesn't use the program, it gets very bad results.

Fixes #4818.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7728043

src/cmd/addr2line/main.c

@@ -11,13 +11,19 @@
 #include <bio.h>
 #include <mach.h>
 
+void
+printusage(int fd)
+{
+	fprint(fd, "usage: addr2line binary\n");
+	fprint(fd, "reads addresses from standard input and writes two lines for each:\n");
+	fprint(fd, "\tfunction name\n");
+	fprint(fd, "\tfile:line\n");
+}
+
 void
 usage(void)
 {
-	fprint(2, "usage: addr2line binary\n");
-	fprint(2, "reads addresses from standard input and writes two lines for each:\n");
-	fprint(2, "\tfunction name\n");
-	fprint(2, "\tfile:line\n");
+	printusage(2);
 	exits("usage");
 }
 
@@ -32,6 +38,11 @@ main(int argc, char **argv)
 	Biobuf bin, bout;
 	char file[1024];
 
+	if(argc > 1 && strcmp(argv[1], "--help") == 0) {
+		printusage(1);
+		exits(0);
+	}
+
 	ARGBEGIN{
 	default:
 		usage();