Fix metal bugs so deathstar.com runs in qemu (#20)

- Remove XD bit in page tables
- Fix cylinder+head+sector arithmetic
- Implement fstat() for serial file descriptors on metal

Here's how to boot an Actually Portable Executable in QEMU:

    make -j12 o//tool/viz/deathstar.com
    qemu-system-x86_64 -serial stdio -fda o//tool/viz/deathstar.com

Here's a screenshot of DEATHSTAR.COM booted in QEMU:
https://justine.lol/cosmopolitan/cosmo-metal-qemu.png

Thus metal support is in much better shape now, but still incomplete.
Only a few system calls have been polyfilled. To figure out which ones
your program needs, simply boot it in the blinkenlights emulator with a
breakpoint, and press CTRL-C to continue to the system call breakpoint.
If it doesn't break then you should be good. (Note: to emulate normally
you can press 'c' and use CTRL-T and ALT-T to tune the speed.)

    m=tiny
    make -j12 SILENT=0 MODE=$m          \
      o/$m/tool/build/blinkenlights.com \
      o/$m/tool/viz/deathstar.com
    o/$m/tool/build/blinkenlights.com   \
      -r -t -b systemfive.linux         \
      o/$m/tool/viz/deathstar.com

Thank @Theldus for the bug report that made this change possible.
Fixes #20 which explains this change further.

libc/calls/fstat.c

@@ -32,7 +32,11 @@ int fstat(int fd, struct stat *st) {
     return weaken(__zipos_fstat)(
         (struct ZiposHandle *)(intptr_t)g_fds.p[fd].handle, st);
   } else if (!IsWindows()) {
-    return fstat$sysv(fd, st);
+    if (!IsMetal()) {
+      return fstat$sysv(fd, st);
+    } else {
+      return fstat$metal(fd, st);
+    }
   } else {
     if (!__isfdkind(fd, kFdFile)) return ebadf();
     return fstat$nt(g_fds.p[fd].handle, st);