Implement floored / and divisor-sign % operators (#745)

Fixes #703

test/asm/div-mod.asm

@@ -0,0 +1,36 @@
+_ASM equ 0
+
+test: MACRO
+; Test RGBASM
+V equs "_ASM +"
+	static_assert \#
+	PURGE V
+; Test RGBLINK
+V equs "_LINK +"
+	assert \#
+	PURGE V
+ENDM
+
+for x, -300, 301
+  for y, -x - 1, x + 2
+    if y != 0
+q = x / y
+r = x % y
+      test (V (q * y + r)) == (V x)
+      test (V (x + y) % y) == (V r)
+      test (V (x - y) % y) == (V r)
+    endc
+  endr
+endr
+
+for x, -300, 301
+  for p, 31
+y = 2 ** p
+r = x % y
+m = x & (y - 1)
+    test (V r) == (V m)
+  endr
+endr
+
+SECTION "LINK", ROM0
+_LINK::