This populates the isolate-independent builtin whitelist with initial
builtins that do not access any isolate-dependent data and thus don't
need the root register at all.

Unlike most other platforms, we can't use a scratch register in the
off-heap trampoline since there's no free register available. The
trampolines on ia32 are thus implemented as pc-relative jumps
(thankfully we can address the entire address space).

Drive-by: Made Code::IsIsolateIndependent consistent with
FinalizeEmbeddedCodeTargets. Code targets are only allowed on some
platforms.

Bug: v8:6666
Change-Id: I0bf02eecba8a099afa7b7c892188cd377cbda840
Reviewed-on: https://chromium-review.googlesource.com/1183224Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55265}