Add test to cover RGBLINK behavior for patch overflow

test/link/patch-overflow.asm

@@ -0,0 +1,3 @@
+section "test", romx
+db startof("test")
+dw startof("test") * 5

test/link/patch-overflow.out

@@ -0,0 +1,3 @@
+error: patch-overflow.asm(3): Value 81920 is not 16-bit
+error: patch-overflow.asm(2): Value 16384 is not 8-bit
+Linking failed with 2 errors