Have rtld query the page size from the kernel

To allow for a dynamic page size on arm64 have the runtime linker
query the kernel for the currentl page size.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34765

(cherry picked from commit e85eaa930862d5b4dc917bc31e8d7254a693635d)