altstack: Use ptrint instead of bare casts

altstack: Use ptrint instead of bare casts

Functions invoked with altstack take a void * parameter.  However, the
test program wants to pass an integer, and so uses the trick of casting
the integer values to (void *) and back again.

The ptrint() module handles exactly this case in a more portable and
(somewhat) typesafe way, so use that instead.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>