Implement pthread barrier.

Bug: 24341262
Change-Id: I5472549e5d7545c1c3f0bef78235f545557b9630
diff --git a/libc/libc.x86.map b/libc/libc.x86.map
index 544ee13..7dc85cc 100644
--- a/libc/libc.x86.map
+++ b/libc/libc.x86.map
@@ -1276,6 +1276,13 @@
     preadv;
     preadv64;
     prlimit; # arm mips x86
+    pthread_barrierattr_destroy;
+    pthread_barrierattr_getpshared;
+    pthread_barrierattr_init;
+    pthread_barrierattr_setpshared;
+    pthread_barrier_destroy;
+    pthread_barrier_init;
+    pthread_barrier_wait;
     pwritev;
     pwritev64;
     scandirat;