add osarch grain and fix debian32 on x86_64

This pull request adds a new grain, ``osarch``, which can be used to detect 32-bit linux installed on 64-bit platform. ``platform.uname()`` returns ``x86_64`` for debian32 when installed on 64-bit hardware, causing 32-bit packages to be improperly identified by salt with a trailing ``:i386``.

See #6607 for more information.