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/ischardev.c

@@ -30,16 +30,11 @@
 textstartup bool32 ischardev(int fd) {
   int olderr;
   struct stat st;
-  if (!IsWindows()) {
-    olderr = errno;
-    if (fstat$sysv(fd, &st) != -1) {
-      return S_ISCHR(st.st_mode);
-    } else {
-      errno = olderr;
-      return false;
-    }
+  olderr = errno;
+  if (fstat(fd, &st) != -1) {
+    return S_ISCHR(st.st_mode);
   } else {
-    return __isfdkind(fd, kFdFile) &&
-           GetFileType(g_fds.p[fd].handle) == kNtFileTypeChar;
+    errno = olderr;
+    return false;
   }
 }