malloc: fix MALLOC_ALIGNMENT constant

The MALLOC_ALIGNMENT constant is supposed to be the default minimum
alignment returned by malloc(). I set it to 8, but this is actually not
what is normally done by recent modern of glibc on Linux, which takes
the maximum of 2*sizeof(size_t) (which is indeed 8) and the
alignof(long double), which is 16 bytes. So to be ABI compatible with
Linux, we should do the same.

Note that while this patch is good for future reference, it doesn't
actually have any practical consequences in the current implementation:
In our current small-malloc implementation, MALLOC_ALIGNMENT is not used
at all, and the alignment is determined just by the size of the allocated
object: A 16-byte object would have 16-byte alignment anyway. For large
allocations the alignment will always be a full page, so again this
change doesn't matter either.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>