sh: Explicit endian selection support.

Previously big endian was simply assumed if little endian was
not set, which led to some cflags ordering issues. There's not
much point to not having a big endian option, so shove one in
a choice and wire it up in the Makefile.

This lets us clean up some of the cflags ordering while we're
at it.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>