Vendored WASI libc sources

These files are copied from WebAssembly/wasi-libc commit 161b3195fc2558d2b1ba3eb9ffae3b2b47407623, which is the wasi-libc submodule revision pinned by WASI SDK 33.

The allowlist is maintained by cargo xtask vendor-wasm-stdlib. Run that command after changing crates/loader/wasi-sdk-version or the pinned WASI_LIBC_REVISION in crates/xtask/src/build_wasm.rs.

The files under string/ and ctype/, as well as LICENSE, are copied verbatim. lib/src/wasm-stdlib/libc.c combines the selected translation units and renames colliding file-local table identifiers; it is maintained by Tree-sitter.