windows: Enable frozen set and sys.exit
py: frozenset() creates an empty frozenset.
travis: Add cross-compilation of mingw port
py: Use mp_arg_check_num in more places.

Updated functions now do proper checking that n_kw==0, and are simpler
because they don't have to explicitly raise an exception.  Down side is
that the error messages no longer include the function name, but that's
acceptable.

Saves order 300 text bytes on x64 and ARM.
Merge pull request #604 from stinos/windows-enablefeat

windows: Enable frozen set and sys.exit
py: Fix clang errors (or otherwise downgrade them to  warning)
unix: Create __bss_start and _end symbols for Mach-O targets.

It's a sort of hack, but I didn't find a cleaner way to do it.
Merge pull request #605 from stinos/travis-mingw

travis: Add cross-compilation of mingw port
py: Add basic implementation of hasattr() function.
showbc: Decode MAP_ADD.
py: Don't expect that type->getiter() always returns iterator, check for NULL.

This is better than forcing each getiter() implementation to raise exception.
py: Fix __len__ special method result handling.

Having both MP_OBJ_NOT_SUPPORTED and MP_OBJ_NULL is arguably confusing.
tests: create result file for test/basics/memoryerror.py . On Mac OS "python3 test/basics/memoryerror.py" never runs out of memory, the process is frozen by the os before.
Merge branch 'master' of https://github.com/micropython/micropython