Implemented via `BCryptGenRandom` on Windows.
Implemented via `getentropy` syscall when available.
Implemented via `/dev/urandom` device as a fallback.
The `/dev/urandom` fallback can be disabled via the `NO_URANDOM` build
flag.
Note: The HTML5 version relies on emscripten file system urandom
device which itself uses the Crypto API when available or the plain
old not crypto-safe `Math.random()` otherwise.
Restore get_entropy.
6b5634b96a