[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: 3a667b9bd8
  libc++abi: 4877063e19
  libc++: 88434fe05f

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

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

libcxx/src/CMakeLists.txt

@@ -176,9 +176,6 @@ split_list(LIBCXX_LINK_FLAGS)
 # Build the shared library.
 if (LIBCXX_ENABLE_SHARED)
   add_library(cxx_shared SHARED ${exclude_from_all} ${LIBCXX_SOURCES} ${LIBCXX_HEADERS})
-  if(COMMAND llvm_setup_rpath)
-    llvm_setup_rpath(cxx_shared)
-  endif()
   target_link_libraries(cxx_shared PUBLIC cxx-headers
                                    PRIVATE ${LIBCXX_LIBRARIES})
   set_target_properties(cxx_shared

libcxxabi/src/CMakeLists.txt

@@ -165,9 +165,6 @@ endif()
 # Build the shared library.
 if (LIBCXXABI_ENABLE_SHARED)
   add_library(cxxabi_shared SHARED ${LIBCXXABI_SOURCES} ${LIBCXXABI_HEADERS})
-  if(COMMAND llvm_setup_rpath)
-    llvm_setup_rpath(cxxabi_shared)
-  endif()
   target_link_libraries(cxxabi_shared PRIVATE ${LIBCXXABI_SHARED_LIBRARIES} ${LIBCXXABI_LIBRARIES})
   if (TARGET pstl::ParallelSTL)
     target_link_libraries(cxxabi_shared PUBLIC pstl::ParallelSTL)

libunwind/src/CMakeLists.txt

@@ -125,9 +125,6 @@ if (LIBUNWIND_ENABLE_SHARED)
   else()
     target_compile_options(unwind_shared PRIVATE -fno-rtti)
   endif()
-  if(COMMAND llvm_setup_rpath)
-    llvm_setup_rpath(unwind_shared)
-  endif()
   target_link_libraries(unwind_shared PRIVATE ${LIBUNWIND_LIBRARIES})
   set_target_properties(unwind_shared PROPERTIES
     CXX_EXTENSIONS OFF