MFC r257637, r257730, r257734, r257777, r257825, r257838, r257873:

Changes to how the aicasm tool is built.  This series of changes results
in the aicasm tool being built as part of the tools stages of world and
kernel builds.

Most of these changes will ultimately be undone when r260401 is MFC'd,
but it will leave in place the new kernel-build-tool machinery (KTMAKE
stuff) in case a new special kernel tool ever comes along.