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/runtime/program_invocation_short_name.S

@@ -28,7 +28,10 @@ program_invocation_short_name:
 
 	.init.start 400,_init_program_invocation_short_name
 	push	%rsi
-	mov	(%r13),%rsi
+	xor	%eax,%eax
+	test	%r12d,%r12d		# argc
+	jz	2f
+	mov	(%r13),%rsi		# argv[0]
 	mov	%rsi,%rcx
 1:	lodsb
 	cmp	$'/,%al
@@ -39,5 +42,5 @@ program_invocation_short_name:
 	jnz	1b
 	xchg	%rcx,%rax
 	pop	%rsi
-	stosq
+2:	stosq
 	.init.end 400,_init_program_invocation_short_name