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>

kernel.h: Move upper_*_bits() and lower_*_bits() to wordpart.h

The wordpart.h header is collecting APIs related to the handling
parts of the word (usually in byte granularity). The upper_*_bits()
and lower_*_bits() are good candidates to be moved to there.

This helps to clean up header dependency hell with regard to kernel.h
as the latter gathers completely unrelated stuff together and slows
down compilation (especially when it's included into other header).

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240214172752.3605073-1-andriy.shevchenko@linux.intel.com
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

kernel.h: removed REPEAT_BYTE from kernel.h

This patch creates wordpart.h and includes it in asm/word-at-a-time.h
for all architectures. WORD_AT_A_TIME_CONSTANTS depends on kernel.h
because of REPEAT_BYTE. Moving this to another header and including it
where necessary allows us to not include the bloated kernel.h. Making
this implicit dependency on REPEAT_BYTE explicit allows for later
improvements in the lib/string.c inclusion list.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Tanzir Hasan <tanzirh@google.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20231226-libstringheader-v6-1-80aa08c7652c@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>