This makes stack checks in WasmCode independent of the underlying
Isolate by loading the limit address from the WasmInstanceObject instead
of embedding it into the instruction stream. It hence removes the last
use of the Isolate field from WasmGraphBuilder.

Additionally this introduces the notion of a "runtime stub" which
represents stub code global to the NativeModule that can be directly
called from each WasmCode in the same module. These stubs can act as
trampolines via which Isolate-independent WasmCode can enter other V8
builtins or runtime functions that remain Isolate-dependent. They will
eventually replace the current "trampoline" in a NativeModule.

R=titzer@chromium.org
BUG=v8:7424

Change-Id: Ie1f5582ee656b1ab7716ea06316d6e21a0268e74
Reviewed-on: https://chromium-review.googlesource.com/1078732
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53487}