Remove machdep.c from files.i386 and put machdep.o in the SYSTEM_OBJS
line of Makefile.i386. Fixes the extra rule that gmake complains about
for machdep.o. This fix is from Joans 0lsson.

Rework the depends and rules for assym.s and genassym so that we now use
the .depend rule for genassym.o such that if you change any header files
that are included by genassym.c the right things happen. This is probably
what has caused more bad kernel builds than any other thing in the
Makefile.i386!