rpc: TestChangeAPI hits an assertion error with gccgo

Bug reported upstream, we should probably attempt a workaround.

https://code.google.com/p/go/issues/detail?id=6761

This seems to be down to the fact that gccgo does not support calling a function that returns a struct via reflect.Call. This fails in the same way:

package main

import (
 "reflect"
)

type aStruct struct{}

func StructReturn () aStruct {
 return aStruct{}
}

func main () {
 f := reflect.ValueOf(StructReturn)
 f.Call([]reflect.Value{})
}

Fails on linux/ppc64el

ubuntu@winton-02:~/src/launchpad.net/juju-core$ go test ./rpc
# launchpad.net/juju-core/rpc_test
/tmp/ccRpWkRW.s: Assembler messages:
/tmp/ccRpWkRW.s:16660: Warning: setting incorrect section attributes for .rodata.__go_tdn_launchpad.net_juju_core_rpc_test.ErrorMethods
/tmp/ccRpWkRW.s:18589: Warning: setting incorrect section attributes for .rodata.__go_tdn_launchpad.net_juju_core_rpc_test.SimpleMethods
/tmp/ccRpWkRW.s:19044: Warning: setting incorrect section attributes for .rodata.__go_tdn_launchpad.net_juju_core_rpc_test..launchpad.net_juju_core_rpc_test.callInfo
panic: interface conversion: interface is nil, not error

goroutine 53 [running]:
rpcreflect.$nested0
        /home/ubuntu/src/launchpad.net/juju-core/rpc/rpcreflect/type.go:147
launchpad.net_juju_core_rpc_rpcreflect.Call.N51_launchpad.net_juju_core_rpc_rpcreflect.MethodCaller
        /home/ubuntu/src/launchpad.net/juju-core/rpc/rpcreflect/value.go:99
launchpad.net_juju_core_rpc.Call.N40_launchpad.net_juju_core_rpc.boundRequest
        /home/ubuntu/src/launchpad.net/juju-core/rpc/server.go:447
launchpad.net_juju_core_rpc.runRequest.pN32_launchpad.net_juju_core_rpc.Conn
        /home/ubuntu/src/launchpad.net/juju-core/rpc/server.go:485
created by launchpad.net_juju_core_rpc.handleRequest.pN32_launchpad.net_juju_core_rpc.Conn
        /home/ubuntu/src/launchpad.net/juju-core/rpc/server.go:417