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/crt/crt.S

@@ -24,21 +24,22 @@
 .section .start,"ax",@progbits
 .source	__FILE__
 
+	nop
+
 /	System Five userspace program entrypoint.
 /
 /	@param	rsp is [n,argv₀..argvₙ₋₁,0,envp₀..,0,auxv₀..,0,..]
 /	@note	FreeBSD is special (see freebsd/lib/csu/amd64/...)
 /	@noreturn
-_start_xnu:
-	movb	$XNU,hostos(%rip)
-	jmp	0f
 _start:	test	%rdi,%rdi
 	cmovnz	%rdi,%rsp
 	jz	0f
 	movb	$FREEBSD,hostos(%rip)
-0:	movslq	(%rsp),%r12			# argc
-	lea	8(%rsp),%r13			# argv
-	lea	24(%rsp,%r12,8),%r14		# envp
+0:	mov	(%rsp),%ebx			# argc
+	lea	8(%rsp),%rsi			# argv
+	lea	24(%rsp,%rbx,8),%rdx		# envp
+	.frame0
+	bofram	9f
 	.weak	idata.iat,idata.iatend
 	ezlea	missingno,ax			# make win32 imps noop
 	ezlea	idata.iat,di
@@ -48,9 +49,21 @@ _start:	test	%rdi,%rdi
 	rep stosq
 	xor	%eax,%eax			# find end of environ
 	or	$-1,%ecx
-	mov	%r14,%rdi
+	mov	%rdx,%rdi
 	repnz scasq
-	mov	%rdi,%r15			# auxv
-	jmp	__executive
-	.endfn	_start,weak,hidden
+	mov	%rdi,%rcx			# auxv
+	mov	%ebx,%edi
+	call	_executive
+9:	.endfn	_start,weak,hidden
+
+	ud2
+
+/	Macintosh userspace program entrypoint.
+/
+/	@param	rsp is [n,argv₀..argvₙ₋₁,0,envp₀..,0,auxv₀..,0,..]
+/	@note	FreeBSD is special (see freebsd/lib/csu/amd64/...)
+/	@noreturn
+_start_xnu:
+	movb	$XNU,hostos(%rip)
+	jmp	0b
 	.endfn	_start_xnu,weak,hidden