Make improvements

- You can now run `make -j8 toolchain` on Windows
- You can now run `make -j` on MacOS ARM64 and BSD OSes
- You can now use our Emacs dev environment on MacOS/Windows
- Fix bug where the x16 register was being corrupted by --ftrace
- The programs under build/bootstrap/ are updated as fat binaries
- The Makefile now explains how to download cosmocc-0.0.12 toolchain
- The build scripts under bin/ now support "cosmo" branded toolchains
- stat() now goes faster on Windows (shaves 100ms off `make` latency)
- Code cleanup and added review on the Windows signal checking code
- posix_spawnattr_setrlimit() now works around MacOS ARM64 bugs
- Landlock Make now favors posix_spawn() on non-Linux/OpenBSD
- posix_spawn() now has better --strace logging on Windows
- fstatat() can now avoid EACCES in more cases on Windows
- fchmod() can now change the readonly bit on Windows

libc/proc/posix_spawn_file_actions_adddup2.c

@@ -26,14 +26,12 @@
  *
  * @param file_actions was initialized by posix_spawn_file_actions_init()
  * @return 0 on success, or errno on error
- * @raise ENOMEM if we require more vespene gas
  * @raise EBADF if 'fildes' or `newfildes` is negative
- * @raise ENOTSUP if `newfildes` isn't 0, 1, or 2 on Windows
+ * @raise ENOMEM if insufficient memory was available
  */
 int posix_spawn_file_actions_adddup2(posix_spawn_file_actions_t *file_actions,
                                      int fildes, int newfildes) {
   if (fildes < 0 || newfildes < 0) return EBADF;
-  if (IsWindows() && newfildes > 2) return ENOTSUP;
   return __posix_spawn_add_file_action(file_actions,
                                        (struct _posix_faction){
                                            .action = _POSIX_SPAWN_DUP2,