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

share/mk/src.opts.mk

@@ -180,6 +180,7 @@ __DEFAULT_NO_OPTIONS = \
     GNU_GREP_COMPAT \
     HESIOD \
     LIBSOFT \
+    LOADER_FORCE_LE \
     NAND \
     OFED \
     OPENLDAP \

sys/boot/Makefile.inc

@@ -1,6 +1,6 @@
 # $FreeBSD$
 
-.include <bsd.own.mk>
+.include <src.opts.mk>
 
 SSP_CFLAGS=
 
@@ -22,3 +22,11 @@ CFLAGS.clang+=  -mfpu=none
 # when this test succeeds rather than require dd to be a bootstrap tool.
 DD_NOSTATUS!=(dd status=none count=0 2> /dev/null && echo status=none) || true
 DD=dd ${DD_NOSTATUS}
+
+.if ${MK_LOADER_FORCE_LE} != "no"
+
+.if ${MACHINE_ARCH} == "powerpc64"
+CFLAGS+=	-mlittle-endian
+.endif
+
+.endif