powerpc/audit: Avoid unneccessary #ifdef in syscall_get_arguments()

Use is_32bit_task() which already handles CONFIG_COMPAT.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ba49cdd574558a0363300c3f6b5b062b397cb071.1629451483.git.christophe.leroy@csgroup.eu