selftests/bpf: Remove hard coded PAGE_SIZE macro.

Remove hard coded PAGE_SIZE.
Add #include <sys/user.h> instead (that works on x86-64 and s390)
and fallback to slow getpagesize() for aarch64.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20240315021834.62988-4-alexei.starovoitov@gmail.com