getpagesize(3): drop support for non-ELF kernels

AT_PAGESZ was introduced with ELF support in 1996 (commit
e1743d02cd14069f69a50bb8a6c626c1c6f47ddd) so we can safely count on
being able to use it to get our page size via elf_aux_info(). As such
we don't need a fallback sysctl query.

Save a few bytes of bss by dropping caching as elf_aux_info() runs
in constant time for a given query.

Reviewed by: kevans, imp, emaste
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42708