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

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.

llvm-svn: 108416