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/calls/ttyname_r.c

@@ -91,7 +91,12 @@ errno_t ttyname_r(int fd, char *buf, size_t size) {
   } else if (IsWindows()) {
     res = sys_ttyname_nt(fd, buf, size);
   } else {
-    res = ENOSYS;
+    // TODO(jart): Use that fstat(dev/ino) + readdir(/dev/) trick.
+    if (strlcpy(buf, "/dev/tty", size) < size) {
+      res = 0;
+    } else {
+      res = ERANGE;
+    }
   }
   errno = e;
   STRACE("ttyname_r(%d, %#.*hhs) → %s", fd, (int)size, buf,