[wasm-split] Use a fresh table when reference types are enabled

[tlively]

Rather than trying to trampoline primary-to-secondary calls through an
existing table, just create a fresh table for this purpose. This ensures
that modifications to the existing tables cannot interfere with
primary-to-secondary calls and conversely that loading the secondary
module cannot overwrite modifications to the tables.

[tlively]

Looks like this uncovered an existing bug in how we handled function references in active segments. I'm glad we're going to get fuzzer coverage for this!

[tlively]

This change will make primary-to-secondary indirect calls slower because they will now trampoline to a second indirect call made through the fresh table. Perhaps we should consider adding a flag to keep the old behavior for use with Emscripten now that reference-types are on by default? (or will be soon?) But we wouldn't be able to fuzz effectively with that flag enabled, so it would be risky.

[kripken]

The extra indirection seems annoying, but yeah, adding a flag would have downsides too. Maybe it's worth measuring though.