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/linkat.c

@@ -41,7 +41,7 @@ int linkat(int olddirfd, const char *oldpath, int newdirfd, const char *newpath,
            int flags) {
   int rc;
   if (IsAsan() &&
-      (!__asan_is_valid(oldpath, 1) || !__asan_is_valid(newpath, 1))) {
+      (!__asan_is_valid_str(oldpath) || !__asan_is_valid_str(newpath))) {
     rc = efault();
   } else if (_weaken(__zipos_notat) &&
              ((rc = __zipos_notat(olddirfd, oldpath)) == -1 ||