selftests/powerpc: Fix load_unaligned_zeropad build failure

selftests/powerpc: Fix load_unaligned_zeropad build failure

This test is userspace code, but uses some kernel headers via symlinks,
and mocks other headers, in order to test load_unaligned_zeropad().

Currently the test fails to build with:

  In file included from load_unaligned_zeropad.c:26:
  word-at-a-time.h:7:10: fatal error: linux/bitops.h: No such file or directory
      7 | #include <linux/bitops.h>

This is due to the recent changes to the kernel headers.

Fix it by symlinking the new wordpart.h, and creating an empty stub for
bitops.h which is all that's needed.

Reported-by: Sachin Sant <sachinp@linux.ibm.com>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Fixes: 66a5c40f60f5 ("kernel.h: removed REPEAT_BYTE from kernel.h")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20240305125644.3315910-1-mpe@ellerman.id.au
Signed-off-by: Kees Cook <keescook@chromium.org>