Other systems mimic the GNU endianness headers, e.g. Cygwin. Instead of
explicitly listing them, just make the Linux case the default case by
moving it into the #else clause.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>