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.