imgact_elf: handle unaligned phdrs

Althought non-compliant, there are binaries which have the phdrs placed
unaligned in the image. Since we have the code to allocate memory for
off-page phdrs, the same code path can be used to handle unaligned
phdrs.

Relax the requirement for both the activated image and interpreter.

PR: 295629
Reviewed by: emaste, markj, olce
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D57498