ld: Permit an Mach-O symbol to be exported in the dynamic symbol table.

Submitting TBR to fix Darwin build.

R=rsc
TBR=rsc
CC=golang-dev
https://golang.org/cl/3912043

src/cmd/ld/ldmacho.c

@@ -630,8 +630,10 @@ ldmacho(Biobuf *f, char *pkg, int64 len, char *pn)
 			s->size = (sym+1)->value - sym->value;
 		else
 			s->size = sect->addr + sect->size - sym->value;
-		s->dynimplib = nil;	// satisfy dynimport
-		s->dynimpname = nil;	// satisfy dynimport
+		if(!s->dynexport) {
+			s->dynimplib = nil;	// satisfy dynimport
+			s->dynimpname = nil;	// satisfy dynimport
+		}
 		if(outer->type == STEXT) {
 			Prog *p;