selftests/seccomp: Get page size from sysconf

The commit fd88d16c ("selftests/seccomp: Be more precise with
syscall arguments.") use PAGE_SIZE directly which lead to build
failure on arm64.

Replace it with generic interface(sysconf(_SC_PAGESIZE)) to fix this
failure.

Build and test successful on x86_64 and arm64.

Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Acked-by: Kees Cook <keescook@chromium.org>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>