Implement -Wtruncation=level (#931)

* Implement -Wtruncation=level

-Wtruncation=0 is the same as the current -Wno-truncation.
-Wtruncation=2 is the same as the current -Wtruncation.
-Wtruncation=1 is the new default; it's less strict, allowing
N-bit values to be between -2**N and 2**N (exclusive).

* Implement generic "parametrized warning" system

* Test more `Wtruncation` variants

Co-authored-by: ISSOtm <eldredhabert0@gmail.com>

src/asm/opt.c

@@ -17,7 +17,8 @@ struct OptStackEntry {
 	bool haltnop;
 	bool optimizeLoads;
 	bool warningsAreErrors;
-	enum WarningState warningStates[NB_WARNINGS];
+	// Don't be confused: we use the size of the **global variable** `warningStates`!
+	enum WarningState warningStates[sizeof(warningStates)];
 	struct OptStackEntry *next;
 };
 
@@ -48,7 +49,7 @@ void opt_L(bool optimize)
 	optimizeLoads = optimize;
 }
 
-void opt_W(char const *flag)
+void opt_W(char *flag)
 {
 	processWarningFlag(flag);
 }