powerpc/syscall: Use is_compat_task()

Instead of hard comparing task flags with _TIF_32BIT, use
is_compat_task(). The advantage is that it returns 0 on PPC32
allthough _TIF_32BIT is always set.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c8094662199337a7200fea9f6e1d1f8b1b6d5f69.1612796617.git.christophe.leroy@csgroup.eu