The existing implementation embedded an isolate-specific pointer to the
thread-in-wasm flag in the wrapper code. However, when the module code
is shared among multiple workers, this can mean that the workers
share the same thread-in-wasm flag.

With this change we load the pointer to the flag at runtime from the
current isolate. Thereby the correct flag is used even when the same
code is executed on different workers.

Note that we could access the right flag address by going through the
root register. However, changing the code generation to use the root
register requires some inconvenient steps:
* Pass the isolate to the pipeline again, which we don't want.
* Change the WasmCallDescriptor to allow the use of the root register
  for wrappers but not for other code.
To avoid these issues, and allow the CL to be easy to merge back, we
got for the changes proposed here.

R=mstarzinger@chromium.org, ishell@chromium.org

Bug: v8:8533
Change-Id: If15565a7ad7cba835cfc1628e7a4d3fdef90a5c0
Reviewed-on: https://chromium-review.googlesource.com/c/1358518
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58044}