tcc: Refactor "compute default outfile name" into libtcc function

tcc: Refactor "compute default outfile name" into libtcc function

Since for upcoming -MD support default _compile_ output file be needed
even when preprocesssing (tcc -E), let's move this code out of one
particular condition block into a common function, so that we could use
it in deps generation code too.

v2:

- As suggested by grischka, moved into libtcc function instead of always
  computing near start of main()
- There is a FIXME about how to return result - I don't want to bother
  callers with allocating temp buffers, not I think it will be a good
  idea to hook default_target to TCCState. Clearly, I'm to used to
  things like std::string and python's str...