powerpc/vdso: Disable building the 32-bit VDSO on little endian

The only little endian configuration we support is ppc64le. As such if
we're building little endian we don't need a 32-bit VDSO, because there
is no 32-bit userspace.

This patch is a fairly ugly mess of #ifdefs, but is the minimal logic
required to disable the 32-bit VDSO. We can hopefully clean up the
result in future with some further refactoring.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --cc arch/powerpc/kernel/Makefile
Simple merge

diff --cc arch/powerpc/kernel/vdso.c
Simple merge

diff --cc arch/powerpc/platforms/Kconfig.cputype
Simple merge