Add simple aligned alloc helper

Add simple aligned alloc helper

We don't have posix_memalign() on some Solaris and Darwin, so just
add a poor mans implementation of it. Outside of arch code where
we have proper posix_memalign(), this is only used to ensure good
alignment of the io_u in the core code.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>