On a box where sizeof(size_t) == 4, C doesn't define
what happens when a size_t value is shifted right by
32 bits, and this caused test_mmap to fail on Windows
in a debug build.  So use different code to break
the size apart depending on how large size_t actually
is.

This looks like an illusion, since lots of code in this
module still appears to assume sizes can't be more
than 32 bits (e.g., the internal _GetMapSize() still
returns an int), but at least test_mmap passes again.