Before WebAssembly#8443, we scanned `ref.func`s in global initizliers early in
`indirectReferencesToSecondaryFunctions` and created trampolines for
them and replaced `ref.func $func`s with `ref.func $trampoline_func` if
`func` was set to move to a secondary module. But in case the global
containing `ref.func $trampoline_func` also ends up moving to the same
secondary module, creating trampoline and using it was not necessary,
because the global can simply use `ref.func $func` because `func` is in
the same secondary module.

To fix this, in WebAssembly#8443, we postponed creating trampolines for `ref.func`s
in global initializers until `shareImportableItems`. This had a problem,
because we end up creating new trampolines late in
`shareImportableItems`. But trampolines were designed to go through
`indirectCallsToSecondaryFunctions` and `setupTablePatching`, so those
late trampolines were invalid, like
```wast
(func $trampoline_foo
  (call $foo)
)
```
when `foo` is in a secondary module. This was supposed to be converted
to a `call_indirect` in `indirectCallsToSecondaryFunctions` and the
table elements were supposed to set up in `setupTablePatching`.

---

To fix the issue, this PR pre-creates trampolines for `ref.func`s in
all global initializers in `indirectReferencesToSecondaryFunctions`,
before `indirectCallsToSecondaryFunctions` and `setupTablePatching`, but
doesn't make `ref.func`s reference them just yet. We make `ref.func`s in
global initializers reference the trampolines only when it is decided
they are not moving to the same secondary module, in
`shareImportableItems`.

But this can leave unused trampolines in the primary module. So in
`cleanupUnusedTrampolines`, we remove unused trampolines.

This increases acx_gallery code size only by 0.6%. If we didn't clean up
the unnecessary trampolines, the increase was 3.6%. This only removes
unused trampoline functions and not placeholder imports and elements
yet, but they don't affect acx_gallery because it uses
`--no-placeholders`.

Fixes WebAssembly#8510. (Both tests fail for the same reason)