tested on win32/64 to pass the tests when enabled

- libtcc.c :
  let tcc define __leading_underscore if enabled
  tcc_add_symbol() : add _ automatically
- tccelf.c : remove tcc_get_symbol_err(), find_c_sym()
  currently symbol length is limited to 256 in several
  places, so we can use a fixed local buffer for now as well.
- win32/lib/crtinit.c : new file for init/fini
- lib/*.S, tests7* : use __leading_underscore
- bt-log.c: this file wont work relibaly if compiled with gcc