Adds extension M instructions: not tested

bench/tests/op/mulh.s

@@ -0,0 +1,18 @@
+# expected: 0000000000000000, 0000000000000000, ffffffffbd5a0000, ffffffffdeacffff
+    .text
+_start:
+    lui x2, 0xdead0
+    lui x3, 0x0
+    addi x4, x0, 0x1
+    addi x5, x0, (-1)
+    lui x6, 0xdead0
+
+    # Mul 0
+    mulh x31, x2, x3
+
+    # Basic multiplications
+    mulh x31, x2, x4
+    mulh x31, x2, x6
+
+    # Overflow
+    mulh x31, x5, x2