Revert "Fix installheaders target's permissions"

The install of headers excludes the support directory, so these chmod
calls fail on non-existent directories, as seen on this bot:

http://lab.llvm.org:8080/green/job/clang-stage1-configure-RA_build/2801/console

This reverts commit r224300.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@224317 91177308-0d34-0410-b5e6-96231b3b80d8