powerpc/64: Make sys_switch_endian() traceable

We weren't using SYSCALL_DEFINE for sys_switch_endian(), which means
it wasn't able to be traced by CONFIG_FTRACE_SYSCALLS.

By using the macro we create the right metadata and the syscall is
visible. eg:

  # cd /sys/kernel/debug/tracing
  # echo 1 | tee events/syscalls/sys_*_switch_endian/enable
  # ~/switch_endian_test
  # cat trace
  ...
  switch_endian_t-3604  [009] ....   315.175164: sys_switch_endian()
  switch_endian_t-3604  [009] ....   315.175167: sys_switch_endian -> 0x5555aaaa5555aaaa
  switch_endian_t-3604  [009] ....   315.175169: sys_switch_endian()
  switch_endian_t-3604  [009] ....   315.175169: sys_switch_endian -> 0x5555aaaa5555aaaa

Fixes: 529d235a ("powerpc: Add a proper syscall for switching endianness")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>