libtcc: cleanup -x<filetype> switch code

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

libtcc.h

@@ -49,10 +49,10 @@ LIBTCCAPI void tcc_undefine_symbol(TCCState *s, const char *sym);
 
 /* add a file (C file, dll, object, library, ld script). Return -1 if error. */
 LIBTCCAPI int tcc_add_file(TCCState *s, const char *filename, int filetype);
-#define TCC_FILETYPE_BINARY 1
-#define TCC_FILETYPE_C      2
-#define TCC_FILETYPE_ASM    3
-#define TCC_FILETYPE_ASM_PP 4
+#define TCC_FILETYPE_BINARY 'b'
+#define TCC_FILETYPE_C      'c'
+#define TCC_FILETYPE_ASM    's'
+#define TCC_FILETYPE_ASM_PP 'S'
 
 /* compile a string containing a C source. Return -1 if error. */
 LIBTCCAPI int tcc_compile_string(TCCState *s, const char *buf);