This directory flattens some of what would be found under
$SRC/cmd/mdb in illumos-gate, where the Makefile.* here
are from a couple different levels in illumos gate:
  cmd/mdb, cmd/mdb/intel, cmd/mdb/sparc

The subdirectories under here are somewhat like:
  cmd/mdb/intel/modules/*
  cmd/mdb/intel/{ia32,amd64}
  cmd/mdb/sparc/{v7,v9}

Didn't want to replicate that whole hierarchy just to
build some mdb modules.

To add a typical intel mdb/kmdb module, you would create:
  mymod/Makefile
  mymod/mymod.c
  mymod/amd64/Makefile
  mymod/ia32/Makefile