audit: define AUDIT_ARCH_OPENRISC

When using audit on OpenRISC, an audit arch is needed. This defines
it and fixes a compile-time bug uncovered in linux-next, likely from a
cut/paste from an arch with 64/32-bit modes that defined arch_arch():
arch/openrisc/kernel/ptrace.c:190:2: error: implicit declaration of function 'audit_arch'

This replaces it with the newly defined AUDIT_ARCH_OPENRISC, since it
is only 32-bit, and currently only operates in big-endian mode.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonas Bonn <jonas@southpole.se>