Fix else working incorrectly from macros

Since the "skip ELSE blocks" variable is global, it used to get carried
over from the macro's `if` to the outer's.

test/asm/if-macro.asm

@@ -0,0 +1,13 @@
+m: macro
+	if 0
+		WARN "3"
+	else
+		WARN "5"
+	endc
+endm
+
+if 1
+	m
+else
+	WARN "12"
+endc