cmd/ld: restore the call graph dump

Before the switch to liblink, the linkers accepted the -c flag
to print the call graph. This change restores the functionality.

This came in handy when I was trying to audit the use of SSE
instructions inside the Plan 9 note handler.

LGTM=rsc
R=golang-codereviews, bradfitz, rsc
CC=golang-codereviews
https://golang.org/cl/73990043

src/cmd/ld/lib.c

@@ -1483,6 +1483,27 @@ undef(void)
 		errorexit();
 }
 
+void
+callgraph(void)
+{
+	LSym *s;
+	Reloc *r;
+	int i;
+
+	if(!debug['c'])
+		return;
+
+	for(s = ctxt->textp; s != nil; s = s->next) {
+		for(i=0; i<s->nr; i++) {
+			r = &s->r[i];
+			if(r->sym == nil)
+				continue;
+			if((r->type == R_CALL || r->type == R_CALLARM) && r->sym->type == STEXT)
+				Bprint(&bso, "%s calls %s\n", s->name, r->sym->name);
+		}
+	}
+}
+
 void
 diag(char *fmt, ...)
 {

src/cmd/ld/lib.h

@@ -178,6 +178,7 @@ void	asmplan9sym(void);
 uint16	be16(uchar *b);
 uint32	be32(uchar *b);
 uint64	be64(uchar *b);
+void	callgraph(void);
 void	cflush(void);
 void	codeblk(int32 addr, int32 size);
 vlong	cpos(void);

src/cmd/ld/pobj.c

@@ -164,6 +164,7 @@ main(int argc, char *argv[])
 	}
 
 	deadcode();
+	callgraph();
 	paramspace = "SP";	/* (FP) now (SP) on output */
 
 	doelf();