posix: re-introduce PTHREAD_RWLOCK_INITIALIZER

Rename the POSIX_RWLOCK_INITIALIZER back to PTHREAD_RWLOCK_INITIALIZER.
This was changed in 70e2b02, but PTHREAD_RWLOCK_INITIALIZER is the
standard name used by external libraries.

Change it back to restore compatibility.

(cherry picked from commit c250c33)

Original-Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>
GitOrigin-RevId: c250c33
Change-Id: Id15074993e3c3044322ecc3a0ac74c9cfd4d8cb9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5602980
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Tested-by: Fabio Baltieri <fabiobaltieri@google.com>
Reviewed-by: Fabio Baltieri <fabiobaltieri@google.com>
Commit-Queue: Fabio Baltieri <fabiobaltieri@google.com>

include/zephyr/posix/pthread.h

@@ -153,7 +153,7 @@ int pthread_condattr_setclock(pthread_condattr_t *att, clockid_t clock_id);
  *
  * Initialize a rwlock with the default rwlock attributes.
  */
-#define POSIX_RWLOCK_INITIALIZER (-1)
+#define PTHREAD_RWLOCK_INITIALIZER (-1)
 
 /*
  * Mutex attributes - type

lib/posix/options/rwlock.c

@@ -86,7 +86,7 @@ struct posix_rwlock *to_posix_rwlock(pthread_rwlock_t *rwlock)
  size_t bit;
  struct posix_rwlock *rwl;
 
- if (*rwlock != POSIX_RWLOCK_INITIALIZER) {
+ if (*rwlock != PTHREAD_RWLOCK_INITIALIZER) {
  return get_posix_rwlock(*rwlock);
  }
 
@@ -116,7 +116,7 @@ int pthread_rwlock_init(pthread_rwlock_t *rwlock,
  struct posix_rwlock *rwl;
 
  ARG_UNUSED(attr);
- *rwlock = POSIX_RWLOCK_INITIALIZER;
+ *rwlock = PTHREAD_RWLOCK_INITIALIZER;
 
  rwl = to_posix_rwlock(rwlock);
  if (rwl == NULL) {