[Bug #21448] Use `getentropy(2)` only on macOS
If this is not a system call, then it is using getrandom (which would
have been tried already), and cannot be used as a replacement for the
random devices.
[Bug #21448] Reorder trials in `fill_random_bytes`
First try dedicated system calls, such as `getrandom` or `getentropy`,
next possible libraries, then fallback to `/dev/urandom`.
Related issues
Bug #21448: Random.urandom may fail to fall back to reading /dev/urandom on Linux < 3.17
merge revision(s) 1181a682a6c314c92686e3701defa1eb44068c4e, d84a811f31a65821642b165d712f380c0cc060e0: [Backport #21448]