Removing HWLOC_SUBDIR from hwloc's 'make clobber'

[ok'd by sung]

With hwloc-1.9 we have only the source code in the repo. Previously we had the
tar and just unpacked that before buildling hwloc. Now that we have only the
source code and not the tar, we don't want to remove the source code when we do
a clobber.


git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23693 3a8e244f-b0f2-452b-bcba-4c88e055c3ca