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/madvise-nt.c

@@ -31,7 +31,7 @@ forceinline typeof(PrefetchVirtualMemory) *GetPrefetchVirtualMemory(void) {
   if (!once) {
     once = true;
     PrefetchVirtualMemory_ = /* win8.1+ */
-        GetProcAddressModule("KernelBase.dll", "PrefetchVirtualMemory");
+        GetProcAddressModule("Kernel32.dll", "PrefetchVirtualMemory");
   }
   return PrefetchVirtualMemory_;
 }
@@ -42,7 +42,7 @@ forceinline typeof(OfferVirtualMemory) *GetOfferVirtualMemory(void) {
   if (!once) {
     once = true;
     OfferVirtualMemory_ = /* win8.1+ */
-        GetProcAddressModule("KernelBase.dll", "OfferVirtualMemory");
+        GetProcAddressModule("Kernel32.dll", "OfferVirtualMemory");
   }
   return OfferVirtualMemory_;
 }