std: move NuttX to use arc4random for random number generation

* 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.

Signed-off-by: Huang Qi <huangqi3@xiaomi.com>

Author: no1wudi

library/std/src/sys/random/mod.rs

@@ -20,6 +20,7 @@ cfg_if::cfg_if! {
         target_os = "rtems",
         target_os = "solaris",
         target_os = "vita",
+        target_os = "nuttx",
     ))] {
         mod arc4random;
         pub use arc4random::fill_bytes;
@@ -44,7 +45,6 @@ cfg_if::cfg_if! {
         target_os = "hurd",
         target_os = "l4re",
         target_os = "nto",
-        target_os = "nuttx",
     ))] {
         mod unix_legacy;
         pub use unix_legacy::fill_bytes;