Add x86_64-linux-gnu emulator

I wanted a tiny scriptable meltdown proof way to run userspace programs
and visualize how program execution impacts memory. It helps to explain
how things like Actually Portable Executable works. It can show you how
the GCC generated code is going about manipulating matrices and more. I
didn't feel fully comfortable with Qemu and Bochs because I'm not smart
enough to understand them. I wanted something like gVisor but with much
stronger levels of assurances. I wanted a single binary that'll run, on
all major operating systems with an embedded GPL barrier ZIP filesystem
that is tiny enough to transpile to JavaScript and run in browsers too.

https://justine.storage.googleapis.com/emulator625.mp4

libc/stubs/mulvti3.S

@@ -18,27 +18,119 @@
 │ 02110-1301 USA                                                               │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/macros.h"
-.align	16
-.text.likely
 .source	__FILE__
+.privileged
+.alignfunc
 
 /	Returns 𝑥*𝑦, aborting on overflow.
 /
-/	TODO(jart): is this ok?
-/
 /	@param	rdi:rsi is int128 𝑥
 /	@param	rdx:rcx is int128 𝑦
 /	@return	rdx:rax is 𝑥*𝑦
 /	@see	-ftrapv
 __mulvti3:
+	push	%rbp
+	mov	%rsp,%rbp
+	push	%rbx
+	push	%rbx
+	push	%r12
+	push	%r13
+	push	%r14
+	push	%r15
+	mov	%rdx,%r10
+	mov	%rdi,%rdx
+	xor	%r11d,%r11d
+	mov	%r10,%rax
+	sar	$63,%rdx
+	sar	$63,%rax
+	cmp	%rsi,%rdx
+	jne	4f
+	cmp	%rcx,%rax
+	jne	5f
+	mov	%rdi,%rax
+	imul	%r10
+	mov	%rax,%r14
+	mov	%rdx,%r8
+	jmp	2f
+5:	mov	%r10,%r12
+	mov	%rcx,%r13
+	mov	%rcx,%r8
+	mov	%rdi,%rbx
+	jmp	6f
+4:	cmp	%rcx,%rax
+	jne	7f
+	mov	%rdi,%r12
+	mov	%rsi,%r13
+	mov	%rsi,%r8
+	mov	%r10,%rbx
+6:	mov	%rdi,%rax
+	mul	%r10
+	mov	%rax,%r14
+	mov	%rbx,%rax
+	mov	%rdx,%r15
+	mul	%r8
+	test	%r8,%r8
+	jns	8f
+	xor	%r8d,%r8d
+	sub	%r8,%rax
+	sbb	%rbx,%rdx
+8:	test	%rbx,%rbx
+	jns	9f
+	sub	%r12,%rax
+	sbb	%r13,%rdx
+9:	mov	%r15,%r8
+	xor	%r9d,%r9d
+	add	%rax,%r8
+	adc	%rdx,%r9
+	mov	%r8,%rdx
+	sar	$63,%rdx
+	cmp	%r9,%rdx
+	je	2f
+	imul	%r10,%rsi
 	mov	%rdi,%rax
-	imul	%rdx,%rsi
 	imul	%rdi,%rcx
-	add	%rsi,%rcx
-	mul	%rdx
-	jc	1f
-	add	%rcx,%rdx
-	jo	1f
+	mul	%r10
+	lea	(%rsi,%rcx),%r8
+	add	%rdx,%r8
+	mov	%rax,%r14
+	jmp	3f
+7:	mov	%rsi,%r8
+	mov	%rcx,%rdx
+	mov	%rdi,%rax
+	imul	%rdi,%rdx
+	imul	%r10,%r8
+	add	%rdx,%r8
+	mul	%r10
+	mov	%rax,%r14
+	lea	1(%rsi),%rax
+	add	%rdx,%r8
+	cmp	$1,%rax
+	ja	3f
+	lea	1(%rcx),%rax
+	cmp	$1,%rax
+	ja	3f
+	cmp	%rcx,%rsi
+	jne	11f
+	cmp	%r14,%r11
+	mov	%r11,%rax
+	sbb	%r8,%rax
+	jl	2f
+	jmp	3f
+11:	test	%r8,%r8
+	js	2f
+3:	mov	$1,%r11d
+2:	test	%r11,%r11
+	je	1f
+	mov	%r8,-8(%rbp)
+	call	__on_arithmetic_overflow
+	mov	-8(%rbp),%r8
+1:	mov	%r14,%rax
+	mov	%r8,%rdx
+	pop	%r15
+	pop	%r14
+	pop	%r13
+	pop	%r12
+	pop	%rbx
+	leave
 	ret
-1:	jmp	__on_arithmetic_overflow
 	.endfn	__mulvti3,globl