Fix and improve currentServiceMethodName() for gccgo

[dimitern]

In goose/testservices/hook/service_gccgo.go there is some logic to extract the method name using the call stack from the runtime package. This works well for the default gc compiler, bug causes test failures with gccgo compiler, e.g. on ppc64el - http://paste.ubuntu.com/10356017/ This happens regularly in Juju CI tests (job: run-unit-tests-trusty-ppc64el-lxc). After looking through the code I found that I can reproduce the same error locally on a trusty amd64 machine, using go test -check.v -check.f TestBootstrapFailsWhenPublicIPError -compiler gccgo: http://paste.ubuntu.com/10356073/

I have a patch in mind which should resolve this, but I'm still testing it as I'm hitting random failures with gccgo as described in this bug https://bugs.launchpad.net/ubuntu/+source/gccgo-go/+bug/1393825