Added checksum_stubs.c from mirage-platform

This is only used for networking, and isn't specific to a Mirage
platform (the Xen and Unix versions should be the same).

We now always use the x86_64-specific optimised version from mirage-xen
on x86_64 (whether Unix or Xen), and always use the generic version from
mirage-unix on other platforms.
When splitting a page, copy the second bit to a new page

We have to do a copy here because Linux dom0 doesn't like getting the
same physical page twice.

See: http://lists.xenproject.org/archives/html/xen-users/2014-07/msg00067.html