Not much to do here. This CL bakes in incompatibility between
poisoning and embedded builtins, since we cannot unconditionally reset
the poison register (which we reused as kRootRegister) as we used to.

It also exposes a bug introduced in [0] where we set
Isolate::c_function to a garbage value.

[0] https://chromium-review.googlesource.com/1185011

Bug: v8:6666
Change-Id: Ia606f5d0e86c7ff68aa2af22acb89c2844519bf5
Reviewed-on: https://chromium-review.googlesource.com/1233255
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56038}