Fix malloc alignment

Fix malloc alignment

In commit 3e0a1f388, Richard tried to fix malloc alignments by using
 alignof (double __attribute_aligned__(sizeof (size_t))).
This doesn't work, since attribute_aligned overrides the alignment
rather than providing a minimum. On C6X, malloc returns four-byte
aligned values rather than the necessary eight-byte alignment.

It's simpler to use a comparison and pick the bigger of the two values,
so that's what I've done.

Signed-off-by: Bernd Schmidt <bernds@codesourcery.com>