Changes

xml: fix statically-allocated opaque backend info array

It was sized for 64bit pointers max, it's not enough on 128bits platforms
such as Capability Hardware Enhanced RISC Instructions (CHERI).

Use something a bit more generic but still a bit hardwired to keep
things simple and statically allocatable on the stack.

Thanks to Jeff Hammond for the report.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

shmem: pad the end of header up to a multiple of pointer size

So that the following topology is well aligned on 128 bits architectures.

No change on 32/64bits architectures.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>