Assert at build time that the target is little-endian.

I’m not sure, but I believe that the code for inline atoms is buggy
on big-endian architectures. It seems like we don’t care enough about
these architectures to make it worth fixing this, but in case someone
ever tries, this should warn them.
Merge pull request #2 from mozilla-servo/endian

Assert at build time that the target is little-endian.