ports/stm32: Update mutex typedef to use k_sem for Zephyr threading.

Changed mp_thread_mutex_t and mp_thread_recursive_mutex_t to use
struct k_sem instead of struct k_mutex. This change is required to
fix GIL deadlock issues with Zephyr threading, as k_mutex has
ownership tracking that prevents cross-thread lock/unlock operations
needed by the GIL.

This matches the reference implementation in ports/zephyr and enables
proper multi-threaded operation with shared locks and thread
arguments.

Signed-off-by: Andrew Leech <[email protected]>

Author: pi-anl

ports/stm32/mpthreadport_zephyr.h

@@ -31,13 +31,15 @@
 
 #include <zephyr/kernel.h>
 
-// Mutex types using Zephyr k_mutex
+// Mutex types using Zephyr k_sem (binary semaphore)
+// Need a binary semaphore so a lock can be acquired on one Python thread
+// and then released on another (required for GIL).
 typedef struct _mp_thread_mutex_t {
-    struct k_mutex handle;
+    struct k_sem handle;
 } mp_thread_mutex_t;
 
 typedef struct _mp_thread_recursive_mutex_t {
-    struct k_mutex handle;
+    struct k_sem handle;
 } mp_thread_recursive_mutex_t;
 
 // Threading functions (implemented in extmod/zephyr_kernel/mpthread_zephyr.c)