cmd/cgo: allow for stdcall decorated dynimport names

cmd/cgo: allow for stdcall decorated dynimport names

To allow for stdcall decorated names on Windows, two changes were needed:
1. Change the symbol versioning delimiter '@' in cgo's dynimport output to a '#', and in cmd/ld when it parses dynimports.
2. Remove the "@N" decorator from the first argument of cgo's dynimport output (PE only).

Fixes #4607.

R=minux.ma, adg, rsc
CC=golang-dev
https://golang.org/cl/7047043