getpagesizes(3): drop support for kernels before 9.0

AT_PAGESIZES and elf_aux_info where added prior to FreeBSD 9.0 in commit
ee235befcb8253fab9beea27b916f1bc46b33147. It's safe to say that a
FreeBSD 15 libc won't work on a 8.x kernel so drop sysctl fallback.

Reviewed by: kevans, imp, emaste
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42707