Bug#1088168: gimp 3 FTBFS on big-endian

Control: retitle -1 gimp 3 FTBFS on big-endian

I'm retitling this bug since gimp 3.0.6 fails in a similar way on the
other big-endian architectures, ppc64 and sparc64. powerpc is also
big-endian but its build tests fail before this point.

m68k is also big-endian but doesn't hit this failure. I don't know
why. Skipping dh_auto_test on s390x is not enough to avoid this
failure.

I'm setting Build-Depends: architecture-is-little-endian now. The only
architecture-independent reverse build dependency is gimp-texturize
which wouldn't be useful without gimp. gimp 3 only built on m68k for
the first time today after I fixed a blocker in gegl. It seems less
wasteful to have builds knowingly hang for hours. It's easy to remove
that Build-Depends restriction and do a test build in an Ubuntu PPA if
someone wants to check if the failure is fixed in a newer gimp
version.

Thank you,
Jeremy Bícha