[libc++] Use generator expression to simplify the CMake code

[ldionne]

Summary

A comment was left for when we would require CMake >= 3, which we do now.
I expect this should be a NFC.

[Mordante]

libcxx/src/CMakeLists.txt

382

Buildkite fails with " Target "cxx" is not an executable or library.", so seem the target is no longer valid.

[ldionne]

Fix incorrect target name

[Mordante]

Since we require CMake > 3 and the build passes, LGTM.

libcxx/src/CMakeLists.txt

install(TARGETS cxx_experimental
     LIBRARY DESTINATION ${LIBCXX_INSTALL_PREFIX}${LIBCXX_INSTALL_LIBRARY_DIR} COMPONENT cxx
     ARCHIVE DESTINATION ${LIBCXX_INSTALL_PREFIX}${LIBCXX_INSTALL_LIBRARY_DIR} COMPONENT cxx
     RUNTIME DESTINATION ${LIBCXX_INSTALL_PREFIX}bin COMPONENT cxx)
 endif()
 
 # NOTE: This install command must go after the cxx install command otherwise
 # it will not be executed after the library symlinks are installed.
 if (LIBCXX_ENABLE_SHARED AND LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
-  # Replace the libc++ filename with $<TARGET_LINKER_FILE:cxx>
-  # after we required CMake 3.0.
-  install(FILES "${LIBCXX_LIBRARY_DIR}/libc++${CMAKE_SHARED_LIBRARY_SUFFIX}"
+  install(FILES "$<TARGET_LINKER_FILE:cxx_shared>"
     DESTINATION ${LIBCXX_INSTALL_PREFIX}${LIBCXX_INSTALL_LIBRARY_DIR}
     COMPONENT libcxx)
 endif()
 
 if (NOT CMAKE_CONFIGURATION_TYPES)
     if(LIBCXX_INSTALL_LIBRARY)
       set(lib_install_target cxx)
     endif()