[libc] Implement getentropy in sys/random (#226501)

Implement the standard POSIX.1-2024 function `getentropy` in
`<sys/random.h>`, bringing `<sys/random.h>` to 100% completion.

Fixes #226458

GitOrigin-RevId: dc887712dab0a5d1f234f9c4a0942c1ff2999a70