ld: Permit an ELF symbol to be exported in the dynamic symbol table.

This permits exporting crosscall2 from the runtime/cgo
package.

R=rsc
CC=golang-dev
https://golang.org/cl/3885041

src/cmd/ld/ldelf.c

@@ -641,8 +641,10 @@ ldelf(Biobuf *f, char *pkg, int64 len, char *pn)
 		s->sub = sect->sym->sub;
 		sect->sym->sub = s;
 		s->type = sect->sym->type | SSUB;
-		s->dynimplib = nil;  // satisfy dynimport
-		s->dynimpname = nil;  // satisfy dynimport
+		if(!s->dynexport) {
+			s->dynimplib = nil;  // satisfy dynimport
+			s->dynimpname = nil;  // satisfy dynimport
+		}
 		s->value = sym.value;
 		s->size = sym.size;
 		s->outer = sect->sym;