This commit goes through and updates various `#[cfg]` as appropriate to
get the wasm64-unknown-unknown target behaving similarly to the
wasm32-unknown-unknown target. Most of this is just updating various
conditions for `target_arch = "wasm32"` to also account for `target_arch
= "wasm64"` where appropriate. This commit also lists `wasm64` as an
allow-listed architecture to not have the `restricted_std` feature
enabled, enabling experimentation with `-Z build-std` externally.

The main goal of this commit is to enable playing around with
`wasm64-unknown-unknown` externally via `-Z build-std` in a way that's
similar to the `wasm32-unknown-unknown` target. These targets are
effectively the same and only differ in their pointer size, but wasm64
is much newer and has much less ecosystem/library support so it'll still
take time to get wasm64 fully-fledged.

These are all stable as-of-now in the WebAssembly specification so any
engine which implements wasm64 will surely implement these features as
well.