powerpc: Remove __arch_swab*

The generic code uses gcc built-ins which work fine so there's no benefit
in implementing our own anymore.

We can't completely remove the ld/st_le* functions as some historical
cruft still uses them, but that's next on the radar

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

diff --cc arch/powerpc/include/asm/swab.h
Simple merge