The interp package was assuming that all targets were little-endian. But
that's not true: we now have a big-endian target (GOARCH=mips).

This fixes the interp package to use the appropriate byte order for a
given target.

The reflect package needs to know the endianness of the system in a few
places. Before this patch, it assumed little-endian systems. But with
GOARCH=mips we now have a big-endian system which also needs to be
supported. So this patch fixes the reflect package to work on big-endian
systems.

Also, I've updated the tests for MIPS: instead of running the
little-endian tests, I've changed it to run the big-endian tests
instead. The two are very similar except for endianness so this should
be fine. To be sure we won't accidentally break little-endian support,
I've kept a single MIPS little-endian test (the CGo test, which doesn't
yet work on big-endian systems anyway).