sys/pal/unix/sync/mutex: Fix Mutex::new() on NuttX

PTHREAD_MUTEX_INITIALIZER is highly configurable on NuttX, making it difficult
to use a hardcoded initializer from libc crate in Rust way.

Instead, call pthread_mutex_init() to initialize the mutex with default attributes, ensuring consistent behavior
with PTHREAD_MUTEX_INITIALIZER.

* Added conditional compilation for NuttX target
* Replaced PTHREAD_MUTEX_INITIALIZER with pthread_mutex_init() for NuttX
* Ensured consistent mutex initialization behavior across platforms

Author: no1wudi

library/std/src/sys/pal/unix/sync/mutex.rs

@@ -9,10 +9,26 @@ pub struct Mutex {
 }
 
 impl Mutex {
+    #[cfg(not(target_os = "nuttx"))]
     pub fn new() -> Mutex {
         Mutex { inner: UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER) }
     }
 
+    #[cfg(target_os = "nuttx")]
+    pub fn new() -> Mutex {
+        // PTHREAD_MUTEX_INITIALIZER is highly configurable on NuttX, so it's
+        // hard to use a hardcoded initializer from libc.
+        // Instead, call pthread_mutex_init() to initialize the mutex with the
+        // default attributes to get the same behavior as PTHREAD_MUTEX_INITIALIZER.
+        let mut mutex = Mutex { inner: UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER) };
+
+        unsafe {
+            libc::pthread_mutex_init(mutex.raw(), core::ptr::null());
+        }
+
+        mutex
+    }
+
     pub(super) fn raw(&self) -> *mut libc::pthread_mutex_t {
         self.inner.get()
     }