aix: switch to libuv's own thread barrier impl

It was pointed out that pthread_barrier_wait() behaves slightly
different from other platforms. Switch to libuv's own thread barrier
for uniformity of behavior. Perhaps we'll do that for more platforms
in the future.

Refs: libuv#2003 (comment)

Author: bnoordhuis

include/uv/unix.h

@@ -134,7 +134,7 @@ typedef pthread_cond_t uv_cond_t;
 typedef pthread_key_t uv_key_t;
 
 /* Note: guard clauses should match uv_barrier_init's in src/unix/thread.c. */
-#if !defined(PTHREAD_BARRIER_SERIAL_THREAD)
+#if defined(_AIX) || !defined(PTHREAD_BARRIER_SERIAL_THREAD)
 /* TODO(bnoordhuis) Merge into uv_barrier_t in v2. */
 struct _uv_barrier {
   uv_mutex_t mutex;

src/unix/thread.c

@@ -49,7 +49,7 @@ STATIC_ASSERT(sizeof(uv_barrier_t) == sizeof(pthread_barrier_t));
 #endif
 
 /* Note: guard clauses should match uv_barrier_t's in include/uv/uv-unix.h. */
-#if !defined(PTHREAD_BARRIER_SERIAL_THREAD)
+#if defined(_AIX) || !defined(PTHREAD_BARRIER_SERIAL_THREAD)
 int uv_barrier_init(uv_barrier_t* barrier, unsigned int count) {
   struct _uv_barrier* b;
   int rc;