Makefile.inc1: override MACHINE for native-xtools

For the final step of the native-xtools target, "everything" is built
with TARGET and TARGET_ARCH set to the architecture we wish to
cross-build for. However, CROSSENV overwrites the values of MACHINE and
MACHINE_ARCH, setting them to be identical to TARGET and TARGET_ARCH.
For native-xtools this is undesirable since we are building binaries to
run on the host architecture, that can cross compile for the target
architecture.

When building native-xtools for RISC-V, this issue manifests as an invalid
argument for "-march". The compiler is invoked with the target triple of the
host architecture, but the CFLAGS inherited from bsd.cpu.mk are that of the
target architecture.

Reviewed by:	imp, bdrewery
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D23838

Makefile.inc1

@@ -2650,6 +2650,7 @@ NXBMAKE+=	XCC="${NXBOBJTOP}/tmp/usr/bin/cc" \
 		XCPP="${NXBOBJTOP}/tmp/usr/bin/cpp"
 .endif
 NXBMAKE+=	${NXBMAKEENV} ${MAKE} -f Makefile.inc1 ${NXBMAKEARGS} \
+		MACHINE=${MACHINE} MACHINE_ARCH=${MACHINE_ARCH} \
 		TARGET=${NXB_TARGET} TARGET_ARCH=${NXB_TARGET_ARCH} \
 		TARGET_TRIPLE=${MACHINE_TRIPLE:Q}
 # NXBDIRS is improperly based on MACHINE rather than NXB_TARGET.  Need to