loader: support for mixed-endianness ELF/loader and POWER8
On POWER8 with current petitpoot, the loader.kboot might be
run as little-endian application. The FreeBSD kernel is
always big-endian, so the load_elf_* routines must be aware
of proper endianness of all fields.
Submitted by: Wojciech Macek <wma@semihalf.com>
Obtained from: Semihalf
Sponsored by: IBM, QCM Technologies
Differential revision: https://reviews.freebsd.org/D12422