cmd/link: add -dumpdep flag to dump linker dependency graph

This is what led to https://golang.org/cl/20763 and
https://golang.org/cl/20765 to shrink binary sizes.

Change-Id: Id360d474e6153cfe32a525b0a720810fd113195b
Reviewed-on: https://go-review.googlesource.com/22392Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/link/internal/ld/deadcode.go

@@ -196,6 +196,13 @@ func (d *deadcodepass) mark(s, parent *LSym) {
 	if s.Attr.ReflectMethod() {
 		d.reflectMethod = true
 	}
+	if flag_dumpdep {
+		p := "_"
+		if parent != nil {
+			p = parent.Name
+		}
+		fmt.Printf("%s -> %s\n", p, s.Name)
+	}
 	s.Attr |= AttrReachable
 	s.Reachparent = parent
 	d.markQueue = append(d.markQueue, s)

src/cmd/link/internal/ld/lib.go

@@ -196,6 +196,7 @@ var (
 	Funcalign          int
 	iscgo              bool
 	elfglobalsymndx    int
+	flag_dumpdep       bool
 	flag_installsuffix string
 	flag_race          int
 	flag_msan          int

src/cmd/link/internal/ld/pobj.go

@@ -90,6 +90,7 @@ func Ldmain() {
 	flag.Var(&Buildmode, "buildmode", "set build `mode`")
 	obj.Flagcount("c", "dump call graph", &Debug['c'])
 	obj.Flagcount("d", "disable dynamic executable", &Debug['d'])
+	flag.BoolVar(&flag_dumpdep, "dumpdep", false, "dump symbol dependency graph")
 	obj.Flagstr("extar", "archive program for buildmode=c-archive", &extar)
 	obj.Flagstr("extld", "use `linker` when linking in external mode", &extld)
 	obj.Flagstr("extldflags", "pass `flags` to external linker", &extldflags)