`std`: include `dlmalloc` for all non-wasi Wasm targets

Currently, building std for a custom Wasm target with an OS other than `unknown` will fail, because `sys/alloc/mod.rs` will attempt to use `sys/alloc/wasm.rs`, the dlmalloc-based allocator used on `wasm32-unknown-unknown`.  However, currently dlmalloc is only pulled in when `target_os = "unknown"`.

Instead, we should make `Cargo.toml` and `alloc/mod.rs` match: either
- disable `wasm.rs` in `alloc/mod.rs` where `not(target_os = "unknown")`, or
- pull in `dlmalloc` for all Wasm targets with  `target_family = "wasm32"` that aren't covered by the [upper branches of `alloc/mod.rs`](https://github.com/rust-lang/rust/blob/main/library/std/src/sys/alloc/mod.rs#L72-L100).

This PR takes the latter approach, because it allows more code to compile without a custom allocator.