KUnit: ffs: Validate all the __attribute_const__ annotations

KUnit: ffs: Validate all the __attribute_const__ annotations

While tracking down a problem where constant expressions used by
BUILD_BUG_ON() suddenly stopped working[1], we found that an added static
initializer was convincing the compiler that it couldn't track the state
of the prior statically initialized value. Tracing this down found that
ffs() was used in the initializer macro, but since it wasn't marked with
__attribute_const__, the compiler had to assume the function might
change variable states as a side-effect (which is not true for ffs(),
which provides deterministic math results).

Validate all the __attibute_const__ annotations were found for all
architectures by reproducing the specific problem encountered in the
original bug report.

Build and run tested with everything I could reach with KUnit:

$ ./tools/testing/kunit/kunit.py run --arch=x86_64 ffs
$ ./tools/testing/kunit/kunit.py run --arch=i386 ffs
$ ./tools/testing/kunit/kunit.py run --arch=arm64 --make_options "CROSS_COMPILE=aarch64-linux-gnu-" ffs
$ ./tools/testing/kunit/kunit.py run --arch=arm --make_options "CROSS_COMPILE=arm-linux-gnueabi-" ffs
$ ./tools/testing/kunit/kunit.py run --arch=powerpc ffs
$ ./tools/testing/kunit/kunit.py run --arch=powerpc32 ffs
$ ./tools/testing/kunit/kunit.py run --arch=powerpcle ffs
$ ./tools/testing/kunit/kunit.py run --arch=m68k ffs
$ ./tools/testing/kunit/kunit.py run --arch=loongarch ffs
$ ./tools/testing/kunit/kunit.py run --arch=s390 --make_options "CROSS_COMPILE=s390x-linux-gnu-" ffs
$ ./tools/testing/kunit/kunit.py run --arch=riscv --make_options "CROSS_COMPILE=riscv64-linux-gnu-" ffs
$ ./tools/testing/kunit/kunit.py run --arch=riscv32 --make_options "CROSS_COMPILE=riscv64-linux-gnu-" ffs
$ ./tools/testing/kunit/kunit.py run --arch=sparc --make_options "CROSS_COMPILE=sparc64-linux-gnu-" ffs
$ ./tools/testing/kunit/kunit.py run --arch=sparc64 --make_options "CROSS_COMPILE=sparc64-linux-gnu-" ffs
$ ./tools/testing/kunit/kunit.py run --arch=alpha --make_options "CROSS_COMPILE=alpha-linux-gnu-" ffs
$ ./tools/testing/kunit/kunit.py run --arch=sh --make_options "CROSS_COMPILE=sh4-linux-gnu-" ffs

Closes: https://github.com/KSPP/linux/issues/364
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250804164417.1612371-17-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>