Correctly set rpath on Mac OS X for executable tools. Note that I am not certain...

Correctly set rpath on Mac OS X for executable tools.  Note that I am not certain this is the best fix; the code immediately above looks confused, as it first checks to see if we are NOT on Darwin and then checks the Darwin version number.  This fix allows c-index-test (in Clang) to run outside of running the regression test suite.  I would appreciate if someone reviewed this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@108416 91177308-0d34-0410-b5e6-96231b3b80d8