This CL changes the WasmExportedFunctionData class to store a direct
ExternalPointer to the wasm::FunctionSig instead of referencing it
through a Foreign. This in turn makes it possible to use a unique
pointer tag for that external pointer when the sandbox is enabled.

Drive-by: move WasmInternalFunction::call_target external pointer to the
end of the object, in line with other external pointer fields.

Bug: v8:10391, v8:12949
Change-Id: Ic3ff622a075c9eaa2f8d8835803437466290c928
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3829086
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82523}