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.

ape/lib/mapimage.c

@@ -30,7 +30,7 @@ textreal static void __map_segment(uint64_t k, uint64_t a, uint64_t b) {
 
 textreal void __map_image(void) {
   __map_segment(PAGE_V | PAGE_U, 0, (uintptr_t)_etext - IMAGE_BASE_VIRTUAL);
-  __map_segment(PAGE_V | PAGE_U | PAGE_RW | PAGE_XD,
+  __map_segment(PAGE_V | PAGE_U | PAGE_RW,
                 (uintptr_t)_etext - IMAGE_BASE_VIRTUAL,
                 (uintptr_t)_end - IMAGE_BASE_VIRTUAL);
 }