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/fchmod.c

@@ -17,18 +17,32 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/calls.h"
+#include "libc/calls/internal.h"
+#include "libc/calls/syscall-nt.internal.h"
 #include "libc/calls/syscall-sysv.internal.h"
 #include "libc/dce.h"
+#include "libc/intrin/strace.internal.h"
 #include "libc/sysv/errfuns.h"
 
 /**
  * Changes file permissions via open()'d file descriptor.
  *
  * @param mode contains octal flags (base 8)
+ * @raise EROFS if `fd` is a `/zip/...` file
  * @asyncsignalsafe
  * @see chmod()
  */
 int fchmod(int fd, uint32_t mode) {
-  // TODO(jart): Windows
-  return sys_fchmod(fd, mode);
+  int rc;
+  if (__isfdkind(fd, kFdZip)) {
+    rc = erofs();
+  } else if (IsLinux() || IsXnu() || IsFreebsd() || IsOpenbsd() || IsNetbsd()) {
+    rc = sys_fchmod(fd, mode);
+  } else if (IsWindows()) {
+    rc = sys_fchmod_nt(fd, mode);
+  } else {
+    rc = enosys();
+  }
+  STRACE("fchmod(%d, %#o) → %d% m", fd, mode, rc);
+  return rc;
 }