py: Use <alloca.h> for alloca()

alloca() is declared in alloca.h which als happens to be included by stdlib.h.
On mingw however it resides in malloc.h only.
So if we include alloca.h directly, and add an alloca.h for mingw in it's port
directory we can get rid of the mingw-specific define to include malloc.h
and the other ports are happy as well.
Ignore .exe files
mingw: Add 'open' as extra builtin and add define MICROPY_MOD_SYS_STDFILES

This one was probable missed when creating mpconfigport.h, but the underlying
code (file.c) gets compiled and works perfectly so no reason to omit it.
objclosure, objcell: Print detailed representation if was requested.

Well, it is bound to "detailed error reporting", but that's closest what we
have now without creating new entities.
Merge pull request #553 from stinos/ignore-exe

Ignore .exe files
Merge pull request #550 from stinos/mingw-open

mingw: Add 'open' as extra builtin
Merge pull request #552 from stinos/mingw-alloca

py: Use <alloca.h> for alloca()
stmhal: Add documentation for RTC class.
windows: alloca.h: Add comment, fix line endings.
Merge branch 'master' of github.com:micropython/micropython