selftests/nolibc: Add `getpagesize(2)` selftest

Test the getpagesize() function. Make sure it returns the correct
value.

Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>