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

dsriram/golang,apparentlymart/golang,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,gnanderson/go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,pombredanne/go-deleteme