This simplifies how WebAssembly runtime stubs call other builtins.
Instead of loading the {AllocateHeapNumber} builtin from the instance,
we can now embed its direct functionality since builtins are always
independent of the Isolate by now.

R=delphick@chromium.org

Change-Id: Ieb22e9406aa825e84482acc82619a33821a43758
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1895565Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64737}