The GtkComposeTable cache is always in big-endian format and is
byteswapped on load for the more common little-endian CPUs, but
init_builtin_table() in GtkIMContextSimple can't byteswap the built-in
data without copying it, which is undesirable. Pregenerate both big-
and little-endian compose data, and compile the correct flavour into
each build of GTK. This fixes failure of the composetable test when
building for a big-endian architecture such as s390x and (traditional,
big-endian) powerpc.

Resolves: #4217


Signed-off-by: Simon McVittie <smcv@debian.org>