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/log/showcrashreports.c

@@ -68,7 +68,7 @@ relegated void RestoreDefaultCrashSignalHandlers(void) {
   int e;
   size_t i;
   sigset_t ss;
-  --__strace;
+  strace_enabled(-1);
   sigemptyset(&ss);
   sigprocmask(SIG_SETMASK, &ss, NULL);
   for (i = 0; i < ARRAYLEN(kCrashSigs); ++i) {
@@ -78,7 +78,7 @@ relegated void RestoreDefaultCrashSignalHandlers(void) {
       errno = e;
     }
   }
-  ++__strace;
+  strace_enabled(+1);
 }
 
 static void FreeSigAltStack(void *p) {