makeFuncStub and methodValueStub are used by reflect as
generic function implementations. Each call might have
different arguments. Extract those arguments from the
closure data instead of assuming it is the same each time.

Because the argument map is now being extracted from the
function itself, we don't need the special cases in reflect.Call
anymore, so delete those.

Fixes an occasional crash seen when stack copying does
not update makeFuncStub's arguments correctly.

Will also help make it safe to require stack maps in the
garbage collector.

Derived from CL 142000044 by khr.

LGTM=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/143890044