Make use of is_32bit_task() and CONFIG_CPU_LITTLE_ENDIAN
to simplify syscall_get_arch().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4be53b9187a4d8c163968f4d224267e41a7fcc33.1629451479.git.christophe.leroy@csgroup.eu