imgact: Make et_dyn_addr part of image_params

This already gets passed around between various imgact_elf functions, so
moving it removes an argument from all those places. A future commit
will make use of this for hwpmc, though, to provide the load base for
PIEs, which currently isn't available to tools like pmcstat.

Reviewed by: kib, markj, jhb
Differential Revision: https://reviews.freebsd.org/D39594