Rollup merge of #143776 - no1wudi:fix, r=tgross35

std: move NuttX to use arc4random for random number generation

arc4random support in libc merged in rust-lang/libc#4464, so:

* Move `target_os = "nuttx"` from unix_legacy to arc4random section
* This aligns NuttX with other POSIX-compliant systems that support arc4random
* Improves random number generation quality on NuttX by using the system's built-in arc4random implementation instead of legacy fallback methods

NuttX supports arc4random_buf which provides better entropy and security compared to the legacy random number generation methods.

Author: fmease