6l: work with OS X nm/otool

6l was skipping emitting the (2 byte) symbol table if there were no imported or exported
symbols. You can't just drop the symbol table entirely - the linker dies if you have
a linkedit section but no table. You can omit the linkedit section or both the linkedit
and the dlyd parts in the right circumstances, but that seems much more risky to me.

R=rsc
CC=golang-dev
https://golang.org/cl/2421042

src/cmd/ld/macho.c

@@ -410,7 +410,7 @@ domacholink(void)
 	uint64 val;
 
 	linkoff = 0;
-	if(nlinkdata > 0) {
+	if(nlinkdata > 0 || nstrtab > 0) {
 		linkoff = rnd(HEADR+textsize, INITRND) + rnd(datsize, INITRND);
 		seek(cout, linkoff, 0);