ld: allow .o files with no symbols

Don't crash in dwarf for foreign functions.

R=ken2
CC=golang-dev
https://golang.org/cl/3576043

src/cmd/ld/dwarf.c

@@ -1834,6 +1834,8 @@ writelines(void)
 
 	for(cursym = textp; cursym != nil; cursym = cursym->next) {
 		s = cursym;
+		if(s->text == P)
+			continue;
 
 		// Look for history stack.  If we find one,
 		// we're entering a new compilation unit

src/cmd/ld/ldelf.c

@@ -484,7 +484,7 @@ ldelf(Biobuf *f, char *pkg, int64 len, char *pn)
 	// load string table for symbols into memory.
 	obj->symtab = section(obj, ".symtab");
 	if(obj->symtab == nil) {
-		diag("%s: elf object has no symbol table", pn);
+		// our work is done here - no symbols means nothing can refer to this file
 		return;
 	}
 	if(obj->symtab->link <= 0 || obj->symtab->link >= obj->nsect) {

src/cmd/ld/ldmacho.c

@@ -541,8 +541,8 @@ ldmacho(Biobuf *f, char *pkg, int64 len, char *pn)
 		goto bad;
 	}
 	if(symtab == nil) {
-		werrstr("no symbol table");
-		goto bad;
+		// our work is done here - no symbols means nothing can refer to this file
+		return;
 	}
 
 	if(c->seg.fileoff+c->seg.filesz >= len) {
@@ -651,7 +651,6 @@ ldmacho(Biobuf *f, char *pkg, int64 len, char *pn)
 		}
 		sym->sym = s;
 	}
-	
 
 	// load relocations
 	for(i=0; i<c->seg.nsect; i++) {