[runtimes] Avoid overwriting the rpath unconditionally

When building the runtimes, it's very important not to add rpaths unless
the user explicitly asks for them (the standard way being CMAKE_INSTALL_RPATH),
or to change the install name dir unless the user requests it (via
CMAKE_INSTALL_NAME_DIR).

llvm_setup_rpath() would override the install_name_dir of the runtimes
even if CMAKE_INSTALL_NAME_DIR was specified to something, which is wrong
and in fact even "dangerous" for the runtimes.

This issue was discovered when trying to build libc++ and libc++abi as
system libraries for Apple, where we set the install name dir to /usr/lib
explicitly. llvm_setup_rpath() would cause libc++ to have the wrong install
name dir, and for basically everything on the system to fail to load.
This was discovered just now because we previously used something closer
to a standalone build, where llvm_setup_rpath() wouldn't exist, and hence
not be used.

This is a revert of the following commits:

  libunwind: 3a667b9bd8b741f5ac1d8d47857140a3d70737fb
  libc++abi: 4877063e195dfcc128451bbf3dd7b03d04d2562f
  libc++: 88434fe05fdb112a33052c4d8a91c9e989cb032d

Those added llvm_setup_rpath() for consistency, so it seems reasonable
to revert.

Differential Revision: https://reviews.llvm.org/D91099

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