imgact_elf: exclude invalid alignment requests

Only accept at most superpage alignment, or if the arch does not have
superpages supported, artificially limit it to PAGE_SIZE * 1024.
This is somewhat arbitrary, and e.g. could change what binaries do
we accept between native i386 vs. amd64 ia32 with superpages disabled,
but I do not believe the difference there is affecting anybody with
real (useful) binaries.

Reported and tested by: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D33359