FreeBSD's make knows about the MACHINE, but not the MACHINE_ARCH unless

FreeBSD's make knows about the MACHINE, but not the MACHINE_ARCH unless
it is built with this defined (which it isn't by default). This change
to sys.mk treats the absence of MACHINE_ARCH as i386 on the assumption
that it will be appropriately defined (as something else) on any other
architecture. When building FreeBSD's make with NetBSD tools, both
MACHINE and MACHINE_ARCH are correctly set (e.g. when bootstrapping
FreeBSD's make on NetBSD/mvme68k, MACHINE=mvme68k and
MACHINE_ARCH=m68k). This isn't really needed for the alpha which
has both defined as 'alpha', but I thought it was worth getting the
distinction between a MACHINE and a MACHINE_ARCH correct now.

Now, shouldn't PC98 have MACHINE=pc98 and MACHINE_ARCH=i386 ??!!