selftests/powerpc: Only test lwm/stmw on big endian

Newer binutils (>= 2.36) refuse to assemble lmw/stmw when building in
little endian mode. That breaks compilation of our alignment handler
test:

  /tmp/cco4l14N.s: Assembler messages:
  /tmp/cco4l14N.s:1440: Error: `lmw' invalid when little-endian
  /tmp/cco4l14N.s:1814: Error: `stmw' invalid when little-endian
  make[2]: *** [../../lib.mk:139: /output/kselftest/powerpc/alignment/alignment_handler] Error 1

These tests do pass on little endian machines, as the kernel will
still emulate those instructions even when running little
endian (which is arguably a kernel bug).

But we don't really need to test that case, so ifdef those
instructions out to get the alignment test building again.

Reported-by: Libor Pechacek <lpechacek@suse.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Libor Pechacek <lpechacek@suse.com>
Link: https://lore.kernel.org/r/20210119041800.3093047-1-mpe@ellerman.id.au