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/calls/chdir.c

@@ -36,7 +36,7 @@
 int chdir(const char *path) {
   int rc;
   GetProgramExecutableName();  // XXX: ugly workaround
-  if (!path || (IsAsan() && !__asan_is_valid(path, 1))) {
+  if (!path || (IsAsan() && !__asan_is_valid_str(path))) {
     rc = efault();
   } else if (!IsWindows()) {
     rc = sys_chdir(path);