KUnit: Introduce ffs()-family tests

KUnit: Introduce ffs()-family tests

Add KUnit tests for ffs()-family bit scanning functions: ffs(), __ffs(),
fls(), __fls(), fls64(), __ffs64(), and ffz(). The tests validate
mathematical relationships (e.g. ffs(x) == __ffs(x) + 1), and test zero
values, power-of-2 patterns, maximum values, and sparse bit patterns.

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-gnueabihf-" 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=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=s390 --make_options "CROSS_COMPILE=s390x-linux-gnu-" 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=mips --make_options "CROSS_COMPILE=mipsel-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

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250804164417.1612371-1-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>