Add arc4random and arc4random_buf to NuttX

* Declare `arc4random` and `arc4random_buf` as extern "C" functions in the NuttX module
* Enable access to system-provided random number generation for NuttX targets
* Aligns NuttX FFI with other Unix platforms that expose these functions

This change allows Rust code targeting NuttX to use `arc4random` and `arc4random_buf` for secure random number generation, improving compatibility and feature parity with other targets.

Author: no1wudi

src/unix/nuttx/mod.rs

@@ -591,4 +591,6 @@ extern "C" {
     pub fn pthread_setname_np(thread: pthread_t, name: *const c_char) -> i32;
     pub fn pthread_getname_np(thread: pthread_t, name: *mut c_char, len: usize) -> i32;
     pub fn getrandom(buf: *mut c_void, buflen: usize, flags: u32) -> isize;
+    pub fn arc4random() -> u32;
+    pub fn arc4random_buf(bytes: *mut c_void, nbytes: usize);
 }