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.

include/asm/macro.hpp

@@ -13,7 +13,7 @@ struct MacroArgs {
 	std::vector<std::shared_ptr<std::string>> args;
 
 	uint32_t nbArgs() const { return args.size() - shift; }
-	std::shared_ptr<std::string> getArg(int32_t i) const;
+	std::shared_ptr<std::string> getArg(int32_t num) const;
 	std::shared_ptr<std::string> getAllArgs() const;
 
 	void appendArg(std::shared_ptr<std::string> arg);