The work to merge libc/machine/ up a dir lost the stub doc targets.
So when libc/ recursed into machine/, it would stop going deeper as
the doc rules were empty. But now that libc/ goes directly into the
libc/machine/$arch/ and those have never had doc stubs, the build
fails. Add a quick hack to the top dir to ignore all machine/$arch/
dirs when generating docs. A follow up series will delete all of
this code as it merges all the doc rules into the top newlib dir.
866de704f3