cmd/ld: permit sym to be both cgo_export_static and cgo_export_dynamic

Fixes SWIG callbacks.  Previously crosscall2 was only
cgo_export_static, despite the use of two #pragma declarations
in runtime/cgo/callbacks.c.

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://codereview.appspot.com/7817048