Consistently handle negative shifted macro args (#2014)

Negative macro arguments count from the end, i.e. `\<-1>`
is equivalent to `\<_NARG>`, even after `shift`ing them.

Negative arguments cannot be used to access shifted values.

test/asm/negative-shifted-macro-args.asm

@@ -0,0 +1,22 @@
+MACRO test
+	static_assert _NARG == 10
+	println _NARG
+	println \1, \<10>, \<-1>, \<-10>
+	println \<999>
+	println \<-999>
+	shift 3
+	println _NARG
+	println \1, \7, \<-1>, \<-7>
+	println \<8>
+	println \<-8>
+	println \<-999>
+	shift 7
+	println _NARG
+	println \1
+	println \<-1>
+	shift -10
+	println _NARG
+	println \<-2_147_483_648>
+ENDM
+
+test "a", "b", "c", "d", "e", "f", "g", "h", "i", "j"