Patch from John Stultz.

This reorganises the subarch files to put all the headers under the
include dir, instead of mixing them up with the C files.  The only
interesting part is the top section where he makes it fall back from the
subarch dir to the default dir if there's no .h file under the subarch
dir.

This means we can create multiple subarches without copying every single
file that any subarch wants into all the directories.  And is much
tidier, IMHO.