unix: Create __bss_start and _end symbols for Mach-O targets.

It's a hack, but can't find a cleaner way to do it.
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.
unix: Add asm statements needed to read registers with clang. Code generated by gcc 4.9.0 is unchanged (same statements, different order).

Both are inefficient, saving unmodified registers on the stack.
Fix some unused variables, and silence a clang warning about initialization override in vmentrytable.h
stmhal: Stop USB before entering DFU by software.
py: Rename MP_OBJ_NOT_SUPPORTED to MP_OBJ_NULL.

See issue #608 for justification.
Merge pull request #607 from Anton-2/osx-clang

Allow compilation of unix port under clang on OS X
intime build
Initial dynamic library support implementation for Windows

- need to export symbols; use exports.def for functions and API define for data
- make some functions global instead of static so they can be exported
- 'import' statement looks for dlls with an `Init` func returning a module obj