Align large shared memory allocations

Attached is a patch that aligns large shared memory allocations beyond
MAXIMUM_ALIGNOF. The reason for this is that Intel's cpus have a fast
path for bulk memory copies that only works with aligned addresses. It's
possible that other cpus have similar restrictions.
With 7.3.4, it achives a 5% performance gain with pgbench. It has no
effect with 7.3.3, because the buffers are already aligned by chance. I
haven't properly tested 7.4cvs yet.

One problem is the "32" - it's arbitrary, it probably belongs into an
arch dependant header file. But where?

--
Manfred