selftests/powerpc: Fix build failure of load_unaligned_zeropad test

Commit 7a5692e6 ("arch/powerpc: provide zero_bytemask() for
big-endian") added a call to __fls() in our word-at-a-time.h. That was
fine for the kernel build but missed the fact that we also use
word-at-a-time.h in a userspace test.

Pulling in the kernel version of __fls() gets messy, so just define our
own, it's unlikely to change often.

Fixes: 7a5692e6 ("arch/powerpc: provide zero_bytemask() for big-endian")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>