Use *NSYNC for POSIX threads locking APIs

Condition variables, barriers, and r/w locks now work very well.

third_party/nsync/README.md

@@ -0,0 +1,21 @@
+# *NSYNC
+
+The `THIRD_PARTY_NSYNC` and `LIBC_THREAD` packages include source code
+from *NSYNC. Here's the latest upstream synchronization point:
+
+    git@github.com:google/nsync
+    ac5489682760393fe21bd2a8e038b528442412a7 (1.25.0)
+    Author: Mike Burrows <m3b@google.com>
+    Date:   Wed Jun 1 16:47:52 2022 -0700
+
+NSYNC uses the Apache 2.0 license. We made the following local changes:
+
+  - Write custom `nsync_malloc_()` so `malloc()` can use *NSYNC.
+
+  - Rewrite `futex()` wrapper to support old Linux kernels and OpenBSD.
+
+  - Normalize sources to Cosmopolitan style conventions; *NSYNC upstream
+    supports dozens of compilers and operating systems, at compile-time.
+    Since Cosmo solves portability at runtime instead, most of the build
+    config toil has been removed, in order to help the NSYNC source code
+    be more readable and hackable.