Use wasm64 for the wasm node when available

[tomaka]

paritytech/smoldot#1789

[tomaka]

I tried compiling for wasm64-unknown-unknown by curiosity.

The code of smoldot compiles just fine (after #1041) with -Z build-std=core,panic_abort,std, except for the fact that getrandom must be substituted with manually calling a binding function.
However when running with node --experimental-wasm-memory64, I get a weird error:

Error while adding chain: CompileError: WebAssembly.compile(): Compiling function #19:"_ZN7smoldot6libp2p10read_write21ReadWrite$LT$TN..." failed: memory.copy[2] expected type i32, found local.get of type i64 @+14623

I don't know whether this is a bug in NodeJS or a miscompilation, but in both cases wasm64 doesn't seem completely ready yet, and it's outside of our control.