Add small constants set, exit() and tests entry/exit

src/constants.asm

@@ -1 +1,23 @@
 section .rodata
+	; syscall
+	NR_read			equ	0
+	NR_write		equ	1
+	NR_exit			equ	60
+
+	; file descriptors
+	FD_stdin		equ	0
+	FD_stdout		equ	1
+	FD_stderr		equ	2
+
+	; ASCII
+	EOS			equ	0
+	TAB			equ	9
+	NL			equ	10
+	DQUO			equ	34
+
+	; Sizes
+	RBP_OFFSET_CALLER	equ	16
+	SIZE_QWORD		equ	8
+	SIZE_DWORD		equ	4
+	SIZE_WORD		equ	2
+	SIZE_BYTE		equ	1