Make terminal ui binaries work well everywhere

Here's some screenshots of an emulator tui program that was compiled on
Linux, then scp'd it to Windows, Mac, and FreeBSD.

https://justine.storage.googleapis.com/blinkenlights-cmdexe.png
https://justine.storage.googleapis.com/blinkenlights-imac.png
https://justine.storage.googleapis.com/blinkenlights-freebsd.png
https://justine.storage.googleapis.com/blinkenlights-lisp.png

How is this even possible that we have a nontrivial ui binary that just
works on Mac, Windows, Linux, and BSD? Surely a first ever achievement.

Fixed many bugs. Bootstrapped John McCarthy's metacircular evaluator on
bare metal in half the size of Altair BASIC (about 2.5kb) and ran it in
emulator for fun and profit.

tool/build/emubin/lispstart.S

@@ -17,11 +17,15 @@
 │ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA                │
 │ 02110-1301 USA                                                               │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+.code16
+.section .start,"ax",@progbits
 
-	.code16
-	.section .start,"ax",@progbits
 _start:	jmp	1f
 1:	ljmp	$0x600>>4,$_begin
+	.type	_start,@function
+	.size	_start,.-_start
+	.globl	_start
+
 _begin:	push	%cs
 	pop	%ds
 	push	%cs
@@ -42,26 +46,24 @@ _begin:	push	%cs
 	mov	$v_sectors+0x0200,%ax
 	int	$0x13
 	xor	%bp,%bp
-	sub	$6,%sp
-	call	main
-	nop
-	.type	_start,@function
-	.size	_start,.-_start
-	.globl	_start
-	.globl	v_sectors
-	.globl	main
+	jmp	main
+	.type	_begin,@function
+	.size	_begin,.-_begin
 
+	.section .start.setjmp,"ax",@progbits
 setjmp:	mov	%sp,%ax
-	stosw				# sp
+	stosw
 	xchg	%ax,%si
-	movsw	%ss:(%si),(%di)		# ip
+	movsw	%ss:(%si),(%di)
 	mov	%bp,%ax
-	stosw				# bp
-	ret
+	stosw
+	ret	$6
 	.type	setjmp,@function
 	.size	setjmp,.-setjmp
 	.globl	setjmp
+	.previous
 
+	.section .start.longjmp,"ax",@progbits
 longjmp:
 	mov	(%di),%sp
 	mov	2(%di),%dx
@@ -72,23 +74,4 @@ longjmp:
 	.type	longjmp,@function
 	.size	longjmp,.-longjmp
 	.globl	longjmp
-
-	.globl	q.syntax
-	.type	q.syntax,@function
-	.globl	q.look
-	.type	q.look,@function
-	.globl	q.globals
-	.type	q.globals,@function
-	.globl	q.index
-	.type	q.index,@function
-	.globl	q.token
-	.type	q.token,@function
-	.globl	q.str
-	.type	q.str,@function
-
-	.globl	boot
-	.type	boot,@function
-	.globl	bss
-	.type	bss,@function
-	.globl	rodata
-	.type	rodata,@function
+	.previous