Added dummy instruction enum

Previously, the assembler used the final instruction in the enum
as a "indicator" for when the direcives start. This brings out a
very very obvious issue where if a developer fails to change the
`IS_DIRECTIVE` macro can make the assembler think that a instruction
that is valid as a directive. Therefore, a new "dummy" has been added
and all directives must be **after** the dummy

libjas/include/instruction.h

@@ -47,7 +47,7 @@ typedef struct instr_encode_table instr_encode_table_t;
  * Please update this macro if more directives and/or instructions
  * are added/supported in this enum list (In the future).
  */
-#define INSTR_DIRECTIVE(i) ((uint8_t)i > (uint8_t)INSTR_SYSCALL)
+#define INSTR_DIRECTIVE(i) ((uint8_t)i > (uint8_t)INSTR_DUMMY)
 
 enum instructions {
   INSTR_MOV,
@@ -83,6 +83,8 @@ enum instructions {
   INSTR_INT,
   INSTR_SYSCALL,
 
+  INSTR_DUMMY,
+
   // Directives:
   INSTR_DIR_WRT_BUF, // A buffer_t to write to