MFC: r268475 (by imp)

  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.
MFC: r268474 and r268476 (by imp)
  - Merge the clang support from i386.
  - Compile boot2 with clang on pc98.

MFC: r272250 (by nyan)
  - Reduce diffs against i386.