stdatomics: tidy & cleanup

stdatomics: tidy & cleanup

- remove any error messages that were just for debugging the
  templates really
- don't use c99 in tcc (array designators etc.)
- remove memory model type (cannot be an own type really)
  and move memory model defines from built-in to stdatomics.h
- apply normal casts to non-pointer atomic_function arguments
- tidy the library support
- add some tests for errors/warnings

Also:
- Makefile: move GIT_HASH stuff from CFLAGS to DEFINES and into
  main section (away from what is included by tests for example)
- tccelf.c/tccgen.c: avoid some warnings with newer GCC