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/fileexists.c

@@ -51,7 +51,7 @@ bool fileexists(const char *path) {
   struct ZiposUri zipname;
   uint16_t path16[PATH_MAX];
   e = errno;
-  if (IsAsan() && !__asan_is_valid(path, 1)) {
+  if (IsAsan() && !__asan_is_valid_str(path)) {
     efault();
     res = false;
   } else if (_weaken(__zipos_open) &&