arm64: account for imported functions when encoding relocation islands

[evacchi]

Fixes #2340

Thanks @davidmdm for reporting the issue. Please do confirm if this solves your problem!

Here's a summary for the lazy. On arm64, when we compute the offsets in a trampoline, the following function is given a mapping between all function refs (basically Indexes in the original Wasm file) and offsets (refToBinaryOffset).

func encodeCallTrampolineIsland(refToBinaryOffset []int, islandOffset int, executable []byte) {
	for i := 0; i < len(refToBinaryOffset); i++ {
		trampolineOffset := islandOffset + trampolineCallSize*i

Wasm function indexes account for all functions, including imported functions. However, we do not need to allocate space for such functions in the "trampoline island". In fact, we do not account for them:

	trampolineInterval, callTrampolineIslandSize, err := machine.CallTrampolineIslandInfo(localFns)

however:

1. because we do not account for them,
2. because relocations are being written after the code of each function has been laid out into the mmap'ed segment,

if we write out of the expected bounds, we end up overwriting the beginning of the function that follows that island. This causes all kind of wacky behavior, because when you invoke that function, it starts with some jumps to "random" places.

The fix is relatively simple, and it can be rewritten in a few ways. Essentially, we need to account for the imported functions, and ignore those indexes when we write the island.

[mathetake]

💯

[davidmdm]

I confirm that this has solved my issue !