cmd/ld: fix syms that are both cgo_import_static & cgo_import_dynamic

This is needed for SWIG when linking in internal mode.  In
internal mode if a symbol was cgo_import_static we used to
forget that it was also cgo_import_dynamic.

R=rsc, r
CC=golang-dev
https://codereview.appspot.com/9080043
runtime/cgo: export symbols in both internal and external link mode

R=rsc, r
CC=golang-dev
https://codereview.appspot.com/9060045
cmd/cgo: correct pragma name in comment

R=rsc, alexb, minux.ma, bradfitz, i.caught.air
CC=golang-dev
https://codereview.appspot.com/9082043