powerpc: Implement arch_xor_unlock_is_negative_byte on 32-bit

powerpc: Implement arch_xor_unlock_is_negative_byte on 32-bit

Simply remove the ifdef.  The assembly is identical to that in the
non-optimised case of test_and_clear_bits() on PPC32, and it's not clear
to me how the PPC32 optimisation works, nor whether it would work for
arch_xor_unlock_is_negative_byte().  If that optimisation would work,
someone can implement it later, but this is more efficient than the
implementation in filemap.c.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>