Allow RSSET to take a negative value (#2167)

test/asm/rs.out

@@ -1,16 +1,20 @@
-a1 = $0
-a2 = $1
-a3 = $3
-_RS = $7
-b1 = $7
-b2 = $8
-b3 = $A
-_RS = $E
-c1 = $2A
-c2 = $2B
-c3 = $2D
-_RS = $31
-d1 = $0
-d2 = $1
-d3 = $3
-_RS = $7
+a1 = 0
+a2 = 1
+a3 = 3
+_RS = 7
+b1 = 7
+b2 = 8
+b3 = 10
+_RS = 14
+c1 = 42
+c2 = 43
+c3 = 45
+_RS = 49
+d1 = 0
+d2 = 1
+d3 = 3
+_RS = 7
+e1 = -10
+e2 = -9
+e3 = -7
+_RS = -3