Merge the clang support from i386. Don't move to clang yet.
Make SERIAL support optional again. Enable it for i386 because a huge
percentage of machines has a 16550. Disable it for pc98 since only a
tiny fraction of them have one. These changes save 293 bytes when
building with clang, but preserves the ability to build with serial if
you really want.  We now have 92 bytes free (412 with the in-tree gcc).
Compile boot2 with clang on pc98.
Now that pc98 no longer needs gcc to compile boot2, remove the special
case and treat it just like i386.