Fix cross-building further.

Fix cross-building further.

Introduce ${TARGET} defaulting to ${MACHINE} which should be set to
whatever your target ${MACHINE} is, and use that with world-related
stages.  That is, to build pc98 on alpha, one now needs to set both
TARGET_ARCH=i386 and TARGET=pc98.

The scope of ${TARGET} is limited to Makefile.inc1 and cross-tools.

In particular, this change was tested to fix:

1.  Cross building of "alpha" on i386.  The breakage was introduced
    by rev. 1.10 to sbin/i386/Makefile (missing <machine/cronyx.h>).

2.  Descending into machine-specific subdirs for a different arch.
    Previously, sbin/i386 and usr.sbin/boot0cfg were descended into
    when cross-building "alpha" or pc98 on i386.

3.  Fixes pc98 cross-building which was horribly broken, caused by
    not setting MACHINE correctly (most ${MACHINE} == pc98 checking
    Makefiles put -DPC98 to CFLAGS).