harness: Use run-time _SC_PAGE_SIZE instead of build-time PAGESIZE
    
    The getconf(1) command is inherently not cross-compilation friendly.
    In addition PAGESIZE depends on the specific system, even within a
    specific arch, so using a hard-coded value is never safe.
    
    Signed-off-by: Guillem Jover <guillem@hadrons.org>
    Signed-off-by: Jeff Moyer <jmoyer@redhat.com>