[POWERPC] Use asm-generic/bitops/find.h in bitops.h

Powerpc and ppc have some code in their bitops.h that is exactly the
same as asm-generic/bitops/find.h.  Include this header instead of the
private implementation.

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Paul Mackerras <paulus@samba.org>