[runtimes] Avoid overwriting the rpath unconditionally

[ldionne]

Summary

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.

[phosek]

LGTM

[hintonda]

LGTM...

[hvdijk]

The corresponding compiler-rt change, D42462 (rG78fd93e0396a19cb89d4b874c7cc42255888df56), should have been reverted as well as part of this change, we still have nonsense RPATHs for those. I am not right now in a position to commit that as obvious myself; if someone else wants to revert it, please do, else I will do so once I am able.

[phosek]

In D91099#2833078, @hvdijk wrote:

The corresponding compiler-rt change, D42462 (rG78fd93e0396a19cb89d4b874c7cc42255888df56), should have been reverted as well as part of this change, we still have nonsense RPATHs for those. I am not right now in a position to commit that as obvious myself; if someone else wants to revert it, please do, else I will do so once I am able.

Done in rG21c008d5a5b1e0c2ec3c1659cff961f4b0ccea2c.

libcxx/src/CMakeLists.txt

 endfunction()
 
 split_list(LIBCXX_COMPILE_FLAGS)
 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 ${LIBCXX_LIBRARIES})
   set_target_properties(cxx_shared
     PROPERTIES
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
       LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
       OUTPUT_NAME   "c++"
       VERSION       "${LIBCXX_ABI_VERSION}.0"

libcxxabi/src/CMakeLists.txt

 
 if (NOT TARGET pstl::ParallelSTL)
   message(STATUS "Could not find ParallelSTL, libc++abi will not attempt to use it but the build may fail if the libc++ in use needs it to be available.")
 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)
   endif()
   set_target_properties(cxxabi_shared
                         PROPERTIES
                           CXX_EXTENSIONS
                             OFF

libunwind/src/CMakeLists.txt

 # Build the shared library.
 if (LIBUNWIND_ENABLE_SHARED)
   add_library(unwind_shared SHARED ${LIBUNWIND_SOURCES} ${LIBUNWIND_HEADERS})
   if(CMAKE_C_COMPILER_ID STREQUAL MSVC)
     target_compile_options(unwind_shared PRIVATE /GR-)
   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 ${LIBUNWIND_LIBRARIES})
   set_target_properties(unwind_shared PROPERTIES
     CXX_EXTENSIONS OFF
     CXX_STANDARD 11
     CXX_STANDARD_REQUIRED ON
     COMPILE_FLAGS "${LIBUNWIND_COMPILE_FLAGS}"
     LINK_FLAGS "${LIBUNWIND_LINK_FLAGS}"
     LINKER_LANGUAGE C