Added enum for cc65 exit codes. replaced stdlib exit code names constants in libsrc with cc65 exit code named constants

libsrc/common/_afailed.c

@@ -17,5 +17,5 @@ void __fastcall__ _afailed (char* file, unsigned line)
 {
     raise (SIGABRT);
     fprintf (stderr, "ASSERTION FAILED IN %s(%u)\n", file, line);
-    exit (2);
+    exit (CC65_EXIT_AFAILED);
 }

libsrc/common/abort.c

@@ -16,7 +16,7 @@ void abort (void)
 {
     raise (SIGABRT);
     fputs ("ABNORMAL PROGRAM TERMINATION\n", stderr);
-    exit (3);
+    exit (CC65_EXIT_ABORT);
 }