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/tinymath/ilogbl.S

@@ -18,38 +18,24 @@
 │ 02110-1301 USA                                                               │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/macros.h"
-.source	__FILE__
 
+/	Returns log₂ₓ exponent part of long double.
+/
+/	@param	𝑥 is long double passed on stack
+/	@return	result in %eax
+/	@note	needs sse3
 tinymath_ilogbl:
+	push	%rbp
+	mov	%rsp,%rbp
 	.profilable
-	sub	$24,%rsp
-	fldt	32(%rsp)
-	fnstcw	14(%rsp)
-	movzwl	14(%rsp),%eax
-	orb	$12,%ah
-	movw	%ax,12(%rsp)
+	fldt	16(%rbp)
 	fxtract
 	fstp	%st
-	fldcw	12(%rsp)
-	fistpl	8(%rsp)
-	fldcw	14(%rsp)
-	movl	8(%rsp),%eax
-	add	$24,%rsp
+	push	%rax
+	fisttpl	(%rsp)
+	mov	(%rsp),%eax
+	leave
 	ret
 	.endfn	tinymath_ilogbl,globl
 	.alias	tinymath_ilogbl,ilogbl
-
-/*
-	TODO(jart)
-.globl	ilogbl
-.type	ilogbl,@function
-ilogbl:	sub	$24,%rsp
-	fldt	32(%rsp)
-	fxtract
-	fstp	%st
-	fisttpl	12(%rsp)
-	movl	12(%rsp),%eax
-	add	$24,%rsp
-	ret
-.size	ilogbl,.-ilogbl
-*/
+	.source	__FILE__