Fix #1247: Adapt getClass for primitives to Scala.js subtyping

Note that the checkfile override for run/t6318_primitives.check for
Scala 2.11.0 and 2.11.1 has been wrongly introduced in
efe48f93214f376d99e6409fe94acba0af45d620. This commit both fixes the
mistake and adapts them for the change.