cmd/compile: when marking REFLECTMETHOD, check for reflect package itself
reflect.Type.Method (and MethodByName) can be used to obtain a
reference of a method by reflection. The linker needs to know
if reflect.Type.Method is called, and retain all exported methods
accordingly. This is handled by the compiler, which marks the
caller of reflect.Type.Method with REFLECTMETHOD attribute. The
current code failed to handle the reflect package itself, so the
method wrapper reflect.Type.Method is not marked. This CL fixes
it.