Compile loader as Little-Endian on PPC64/POWER8

Add flag to the makefile to allow loader compilation as
Little-Endian 32-bit executable.
Usage:

make WITH_LOADER_FORCE_LE=yes -C sys/boot all

Submitted by: Wojciech Macek <wma@freebsd.org>
Reviewed by: imp, nwhitehorn
Obtained from: Semihalf
Sponsored by: QCM Technologies
Differential revision: https://reviews.freebsd.org/D12421