Add a warning() function, similiar to yyerror()

This function produces a similar output to the other error handlers, including printing to stderr, and including a stack trace. However, ‘warning’ is displayed instead of ‘ERROR’, and the compilation does not fail.

This function is now used for the deprecation warnings, ensuring that these errors can be found.

include/asm/main.h

@@ -27,6 +27,7 @@ extern void opt_Parse(char *s);
 
 noreturn void fatalerror(const char *fmt, ...);
 void yyerror(const char *fmt, ...);
+void warning(const char *fmt, ...);
 
 #define	YY_FATAL_ERROR fatalerror