This CL ensures that the InterpreterEntryTrampoline, as well as
InterpreterPushArgsThenCall and InterpreterPushArgs preserve
the kRootRegister (ebx).

Bug: v8:6666
Change-Id: I1e5b63f1002ffbe4dac84f039f373b6b77e67d8a
Reviewed-on: https://chromium-review.googlesource.com/1233793Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56047}