Enable getrandom support for Emscripten

[guybedford]

This enables SystemRandom on wasm32-unknown-emscripten.

getrandom 0.4 implements Emscripten via libc getentropy, which Emscripten's runtime backs with crypto.getRandomValues, so this is the environment's CSPRNG rather than a custom or rdrand implementation, matching the criterion for the existing target_os list.

• Adds target_os = "emscripten" to the SystemRandom cfg in src/rand.rs.

No build.rs changes were needed; the existing wasm32 C build works as-is with CC=emcc AR=emar.

Tested locally with Emscripten 6.0.10 and Node 22: cargo test --target wasm32-unknown-emscripten with CARGO_TARGET_WASM32_UNKNOWN_EMSCRIPTEN_RUNNER=node passes the full suite including rand_tests and doctests.