libtcc: cleanup -x<filetype> switch code

Abusing filename[0] as type is just too much of a hack.
-- From 0536407204

tcc.h

@@ -850,7 +850,7 @@ struct TCCState {
 #endif
 
     /* used by main and tcc_parse_args only */
-    char **files; /* files seen on command line */
+    struct filespec **files; /* files seen on command line */
     int nb_files; /* number thereof */
     int nb_libraries; /* number of libs thereof */
     char *outfile; /* output filename */
@@ -863,6 +863,10 @@ struct TCCState {
     ParseArgsState *parse_args_state;
 };
 
+struct filespec {
+    char type, name[1];
+};
+
 /* The current value can be: */
 #define VT_VALMASK   0x003f  /* mask for value location, register or: */
 #define VT_CONST     0x0030  /* constant in vc (must be first non register value) */