powerpc: Add a #define for aligning to a long-sized boundary

Add a #define for aligning to a long-sized boundary. It would be nice
to use sizeof(long) for this, but that requires generating the value
with asm-offsets.c, and asm-offsets.c includes asm-compat.h and we
descend into some sort of recursive include hell.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>