Allow OPT to modify -W

Warning flags are processed individually;
PUSHO and POPO (re)store all the warning states.

include/asm/opt.h

@@ -15,6 +15,7 @@ void opt_B(char chars[2]);
 void opt_G(char chars[4]);
 void opt_P(uint8_t fill);
 void opt_L(bool optimize);
+void opt_W(char const *flag);
 void opt_Parse(char const *option);
 
 void opt_Push(void);