tcc -E: Let output_default be <file>.o instead of a.out

This affectes where `tcc -E -MD file.c` will place generated dependency
information -- previously, for `tcc -E` output_default was a.out, and so
deps were put into a.d .

Avoid this behaviour, by treating `tcc -E` as `tcc -c` with respect to
output_default computation.

This will not hurt anything else (preprocessor outputs to either stdout,
or to explicitely given (-o <file>) destination, so no default filename
is used here), and on the other hand `tcc -E -MD file.c` now puts
dependencies into file.d (the same behaviour as for gcc -E).

v2:

- restructured condition a bit to make it more clear

libtcc.c

@@ -1583,7 +1583,9 @@ LIBTCCAPI const char *tcc_default_target(TCCState *s)
         strcpy(ext, ".exe");
     else
 #endif
-    if (s->output_type == TCC_OUTPUT_OBJ && !s->reloc_output && *ext)
+    if (( (s->output_type == TCC_OUTPUT_OBJ && !s->reloc_output) ||
+          (s->output_type == TCC_OUTPUT_PREPROCESS) )
+        && *ext)
         strcpy(ext, ".o");
     else
         pstrcpy(outfile_default, sizeof(outfile_default), "a.out");