cmd/ld: don't automatically mark symbols created by -X as reachable

This fixes the bug in which the linker reports "missing Go
type information" when a -X option refers to a symbol that is
not used.

Fixes #8821.

LGTM=rsc
R=rsc, r
CC=golang-codereviews
https://golang.org/cl/151000043

src/cmd/ld/data.c

@@ -620,6 +620,7 @@ addstrdata(char *name, char *value)
 {
 	LSym *s, *sp;
 	char *p;
+	uchar reachable;
 
 	p = smprint("%s.str", name);
 	sp = linklookup(ctxt, p, 0);
@@ -630,13 +631,17 @@ addstrdata(char *name, char *value)
 	s = linklookup(ctxt, name, 0);
 	s->size = 0;
 	s->dupok = 1;
+	reachable = s->reachable;
 	addaddr(ctxt, s, sp);
 	adduint32(ctxt, s, strlen(value));
 	if(PtrSize == 8)
 		adduint32(ctxt, s, 0);  // round struct to pointer width
 
-	// in case reachability has already been computed
-	sp->reachable = s->reachable;
+	// addstring, addaddr, etc., mark the symbols as reachable.
+	// In this case that is not necessarily true, so stick to what
+	// we know before entering this function.
+	s->reachable = reachable;
+	sp->reachable = reachable;
 }
 
 vlong

src/cmd/ld/lib.c

@@ -222,8 +222,10 @@ loadlib(void)
 		// Since we are simulating the import, we have to provide this string.
 		cgostrsym = "go.string.\"runtime/cgo\"";
 		if(linkrlookup(ctxt, cgostrsym, 0) == nil) {
+			s = linklookup(ctxt, cgostrsym, 0);
+			s->type = SRODATA;
+			s->reachable = 1;
 			addstrdata(cgostrsym, "runtime/cgo");
-			linklookup(ctxt, cgostrsym, 0)->type = SRODATA;
 		}
 	}
 

test/linkx.go

-// $G $D/$F.go && $L -X main.tbd hello -X main.overwrite trumped $F.$A && ./$A.out
+// $G $D/$F.go && $L -X main.tbd hello -X main.overwrite trumped -X main.nosuchsymbol neverseen $F.$A && ./$A.out
 
 // NOTE: This test is not run by 'run.go' and so not run by all.bash.
 // To run this test you must use the ./run shell script.