When run under gccgo, the test looks for the name that gccgo gives to
a thunk method. This name is not normally visible, but can be seen
when using reflect.FuncForPC as this code does. That name changed in
https://golang.org/cl/89555. Change the test to work with both the
old name "$thunk0" and the new name "thunk0".
Fixesgolang/go#28669
As a first step to remove the hard ties between `tplimpl` and the different namespace packages.
The `lang` package is used as the first example use case.
See #3042