Added 'sim6502' and 'sim65C02' targets.

The targets allow to run cc65 programs in the sim65 exection
einvironment. As there are no "real" i/o facilities there's no
need for header files. Paravirtualized entry points are mapped
to $FFF0 ff. There's a large cc65 progam area from $0200-$FFEF.

The binary format includes a one-byte header indicating the required
execution environment: The value 0 means 6502 and the value 1
means 65C02. The load adress for the binary is fixed to $0200.

Note: Running sim65C02 programs currently doesn't work bcause
sim65 doesn't actually implement 65C02 opcodes.

src/ca65/main.c

@@ -283,6 +283,14 @@ static void SetSys (const char* Sys)
             NewSymbol ("__LYNX__", 1);
             break;
 
+        case TGT_SIM6502:
+            NewSymbol ("__SIM6502__", 1);
+            break;
+
+        case TGT_SIM65C02:
+            NewSymbol ("__SIM65C02__", 1);
+            break;
+
         default:
             AbEnd ("Invalid target name: `%s'", Sys);