cmd/gc: document -pack flag

R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/55140044

src/cmd/gc/doc.go

@@ -37,6 +37,8 @@ Substitute 6g with 8g or 5g where appropriate.
 Flags:
 	-o file
 		output file, default file.6 for 6g, etc.
+	-pack
+		write a package file rather than an object file
 	-e
 		normally the compiler quits after 10 errors; -e prints all errors
 	-p path