* -dt now with lowercase t

* test snippets now separated by real preprocessor statements
  which is valid C also for other compilers

    #if defined test_xxx
       < test snippet x >
    #elif defined test_yyy
       < test snippet y >
    #elif ...
    #endif

* simpler implementation, behaves like -run if no 'test_...' macros
  are seen, works with -E too

* for demonstration I combined some of the small tests for errors
  and warnings (56..63,74) in "60_errors_and_warnings.c"

Also:
* libtcc.c:
  put tcc_preprocess() and tcc_assemble() under the setjmp clause
  to let them return to caller after errors.  This is for -dt -E.
* tccgen.c:
  - get rid of save/restore_parse_state(), macro_ptr is saved
    by begin_macro anyway, now line_num too.
  - use expr_eq for parsing _Generic's controlling_type
  - set nocode_wanted with const_wanted. too, This is to keep
    VT_JMP on vtop when parsing preprocessor expressions.
* tccpp.c: tcc -E: suppress trailing whitespace from lines with
  comments (that -E removes) such as
       NO_GOTPLT_ENTRY,\t    /* never generate ... */