utils.[ch]: add fence_get_page_size()

utils.[ch]: add fence_get_page_size()

Add a function to get the page size used for memory fence purposes, and
use it everywhere where getpagesize() was used.

This offers a single point in code to override the page size, in case
one wants to experiment how the tests work with a higher page size than
what the developer's machine has.

This also offers a clean API, without adding #ifdefs, to tests for
checking the page size.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Ben Avison <bavison@riscosopen.org>