Allow negative values to count macro arguments from the end (#1670)

test/asm/negative-macro-args.asm

@@ -0,0 +1,21 @@
+MACRO mac
+	for i, -1, -_NARG - 1, -1
+		println "{d:i}: \<i> == \<{d:i}>"
+	endr
+	; error cases
+	def i = 0
+	println "{d:i}: \<i> == \<{d:i}>"
+	def i = -_NARG - 1
+	println "{d:i}: \<i> == \<{d:i}>"
+	def i = $7fff_ffff
+	println "{d:i}: \<i> == \<{d:i}>"
+	; signed/unsigned difference error cases
+	def i = $8000_0000
+	println "{d:i}: \<i> == \<{d:i}>"
+	println "{u:i}: \<i> == \<{u:i}>"
+	def i = $ffff_ffff
+	println "{d:i}: \<i> == \<{d:i}>"
+	println "{u:i}: \<i> == \<{u:i}>"
+ENDM
+
+mac A, B, C, D, E, F, G