Add tests for stage 7

bench/tests/op/add.s

@@ -0,0 +1,16 @@
+# expected: dead0000, dead0001, 00000000
+    .text
+_start:
+    lui x2, 0xdead0
+    lui x3, 0x0
+    lui x4, 0x1
+    addi x5, x0, (-1)
+
+    # Add 0
+    add x31, x2, x3
+
+    # Basic addition
+    add x31, x2, x4
+
+    # Overflow
+    add x31, x5, x4