sparc: Always set AUDIT_ARCH, not just on sparc64.

Without fancy makefile rules it's not straightforward
to prevent both arch/sparc/kernel/audit.o and lib/audit.o
from both being used on sparc32.

Since arch/sparc/kernel/audit.c is identical to lib/audit.c
except some CONFIG_COMPAT protected sections of code, just
use it on sparc32 too as that's the simplest way to fix
this.

Signed-off-by: David S. Miller <davem@davemloft.net>