Make more threading improvements

- ASAN memory morgue is now lockless
- Make C11 atomics header more portable
- Rewrote pthread keys support to be lockless
- Simplify Python's unicode table unpacking code
- Make crash report write(2) closer to being atomic
- Make it possible to strace/ftrace a single thread
- ASAN now checks nul-terminated strings fast and properly
- Windows fork() now restores TLS memory of calling thread

libc/sock/sendmsg.c

@@ -80,7 +80,7 @@ ssize_t sendmsg(int fd, const struct msghdr *msg, int flags) {
   }
 
 #if defined(SYSDEBUG) && _DATATRACE
-  if (__strace > 0) {
+  if (__strace > 0 && strace_enabled(0) > 0) {
     kprintf(STRACE_PROLOGUE "sendmsg(");
     if ((!IsAsan() && kisdangerous(msg)) ||
         (IsAsan() && !__asan_is_valid(msg, sizeof(*msg)))) {