openrisc: pass endianness info to sparse

openrisc is big-endian only but sparse assumes the same endianness
as the building machine.
This is problematic for code which expect __BYTE_ORDER__ being
correctly predefined by the compiler which sparse can then
pre-process differently from what gcc would, depending on the
building machine endianness.

Fix this by letting sparse know about the architecture endianness.

To: Jonas Bonn <jonas@southpole.se>
To: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
To: Stafford Horne <shorne@gmail.com>
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>