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/macros.inc

@@ -31,6 +31,9 @@
 .macro	.text.startup
 	.section .text.startup,"ax",@progbits
 .endm
+.macro	.text.exit
+	.section .text.exit,"ax",@progbits
+.endm
 .macro	.firstclass
 	.section .text.hot,"ax",@progbits
 .endm
@@ -85,6 +88,14 @@
 	.section .rodata.cst64,"aM",@progbits,64
 	.align	64
 .endm
+.macro	.tdata
+	.section .tdata,"awT",@progbits
+	.align	4
+.endm
+.macro	.tbss
+	.section .tdata,"awT",@nobits
+	.align	4
+.endm
 
 /	Mergeable NUL-terminated UTF-8 string constant section.
 /
@@ -175,11 +186,16 @@
  .endif
 .endm
 
+/	Custom emulator instruction for bottom stack frame.
+.macro	bofram	endfunc:req
+	.byte	0x0f,0x1f,0x45,\endfunc-.	# nopl disp8(%rbp)
+.endm
+
 /	Overrides LOOP instruction.
 /	With its mop-Fusion Mexican equivalent.
 /	Thus avoiding 3x legacy pipeline slowdown.
 .macro	loop	label:req
-	.byte	0x83,0xe9,0x01		# sub $1,%ecx
+	.byte	0x83,0xe9,0x01			# sub $1,%ecx
 	jnz	\label
 .endm