Fix align_malloc on Linux (#645)

posix_memalign requires alignment to be a power of two and a multiple
of sizeof(void*). All powers of two greater than sizeof(void*) are
multiples of sizeof(void*) so we only need to make sure sizeof(void*)
is the minimum value passed to posix_memalign.

Fixes #644

Signed-off-by: Kevin Petit <kevin.petit@arm.com>