DRTVWR-418: Compute ADDRESS_SIZE from arch if not specified.

Migrate the logic formerly used only for LINUX to detect whether ADDRESS_SIZE
is set to 32 or 64, and if not, detect a default for the platform. But instead
of using uname -m, use python's platform.machine().

On Windows, stop forcing ARCH to i686 and ADDRESS_SIZE to 32.

On Mac, reset default to x86_64 instead of i386; stop forcing ADDRESS_SIZE to
32.