Improve cancellations, randomness, and time

- Exhaustively document cancellation points
- Rename SIGCANCEL to SIGTHR just like BSDs
- Further improve POSIX thread cancellations
- Ensure asynchronous cancellations work correctly
- Elevate the quality of getrandom() and getentropy()
- Make futexes cancel correctly on OpenBSD 6.x and 7.x
- Add reboot.com and shutdown.com to examples directory
- Remove underscore prefix from awesome timespec_*() APIs
- Create assertions that help verify our cancellation points
- Remove bad timespec APIs (cmp generalizes eq/ne/gt/gte/lt/lte)

libc/errno.h

@@ -8,7 +8,8 @@ COSMOPOLITAN_C_START_
  * @see libc/sysv/consts.sh for numbers
  */
 
-#if defined(__GNUC__) && defined(__MNO_RED_ZONE__) && !defined(__STRICT_ANSI__)
+#if defined(__GNUC__) && defined(__x86_64__) && defined(__MNO_RED_ZONE__) && \
+    !defined(__STRICT_ANSI__)
 #define errno                                                           \
   (*({                                                                  \
     errno_t *_ep;                                                       \