Changes to WebAssembly targets and handling undefined symbols

Rust is removing the --allow-undefined flag from WebAssembly target linking, changing how undefined symbols are handled. The --allow-undefined flag has historically allowed undefined symbols in WebAssembly binaries to be treated as imports rather than linking errors. This change aligns WebAssembly behavior with native platforms, which treat undefined symbols as compilation errors by default. Common issues include typos in symbol names, missing library compilations, and unexpected imports in final WebAssembly modules that break downstream tools like wasm-bindgen. Most users won't be affected as unresolved imports typically cause runtime failures anyway; the change mainly improves early error detection and diagnostic clarity.

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