[runtimes] Fix link order of system librarires on Apple platforms (#66940)

On Apple platforms, we always support the -nostdlib++ flag. Hence, it is
not necessary to manually link against system libraries. In fact, doing
so causes us to link against libSystem explicitly, which messes up with
the order of libraries we should use. Indeed:

   Before patch, using the system unwinder (LIBCXXABI_USE_LLVM_UNWINDER = OFF)
   ===========================================================================
   $ otool -L lib/{libc++.1.dylib,libc++abi.1.dylib,libunwind.1.dylib}
   lib/libc++.1.dylib:
         @rpath/libc++.1.dylib
         /usr/lib/libSystem.B.dylib
         @rpath/libc++abi.1.dylib
   lib/libc++abi.1.dylib:
         @rpath/libc++abi.1.dylib
         /usr/lib/libSystem.B.dylib
   lib/libunwind.1.dylib:
         @rpath/libunwind.1.dylib
         /usr/lib/libSystem.B.dylib

   After patch, using the system unwinder (LIBCXXABI_USE_LLVM_UNWINDER = OFF)
   ===========================================================================
   $ otool -L lib/{libc++.1.dylib,libc++abi.1.dylib,libunwind.1.dylib}
   lib/libc++.1.dylib:
         @rpath/libc++.1.dylib
         @rpath/libc++abi.1.dylib
         /usr/lib/libSystem.B.dylib
   lib/libc++abi.1.dylib:
         @rpath/libc++abi.1.dylib
         /usr/lib/libSystem.B.dylib
   lib/libunwind.1.dylib:
         @rpath/libunwind.1.dylib
         /usr/lib/libSystem.B.dylib

   Before patch, with the LLVM unwinder (LIBCXXABI_USE_LLVM_UNWINDER = ON)
   =======================================================================
   $ otool -L lib/{libc++.1.dylib,libc++abi.1.dylib,libunwind.1.dylib}
   lib/libc++.1.dylib:
         @rpath/libc++.1.dylib
         /usr/lib/libSystem.B.dylib
         @rpath/libc++abi.1.dylib
         @rpath/libunwind.1.dylib
   lib/libc++abi.1.dylib:
         @rpath/libc++abi.1.dylib
         /usr/lib/libSystem.B.dylib
         @rpath/libunwind.1.dylib
   lib/libunwind.1.dylib:
         @rpath/libunwind.1.dylib
         /usr/lib/libSystem.B.dylib

   After patch, with the LLVM unwinder (LIBCXXABI_USE_LLVM_UNWINDER = ON)
   ======================================================================
   $ otool -L lib/{libc++.1.dylib,libc++abi.1.dylib,libunwind.1.dylib}
   lib/libc++.1.dylib:
         @rpath/libc++.1.dylib
         @rpath/libc++abi.1.dylib
         @rpath/libunwind.1.dylib
         /usr/lib/libSystem.B.dylib
   lib/libc++abi.1.dylib:
         @rpath/libc++abi.1.dylib
         @rpath/libunwind.1.dylib
         /usr/lib/libSystem.B.dylib
   lib/libunwind.1.dylib:
         @rpath/libunwind.1.dylib
         /usr/lib/libSystem.B.dylib

As we can see, libSystem appears before the just-built libraries before
the patch, which causes the libunwind.dylib bundled in libSystem.dylib
to be used instead of the just-built libunwind.dylib.

We didn't notice the issue until recently when I tried to update the
macOS CI builders to macOS 13.5, where it is necessary to use the right
libunwind library (the exact reason still needs to be investigated).

2 files changed, 1 insertions, 5 deletions

diff --git a/libcxxabi/cmake/config-ix.cmake b/libcxxabi/cmake/config-ix.cmake
index f4ee894..702fe7d 100644
--- a/libcxxabi/cmake/config-ix.cmake
+++ b/libcxxabi/cmake/config-ix.cmake
@@ -95,11 +95,9 @@ if(FUCHSIA)
   set(LIBCXXABI_HAS_PTHREAD_LIB NO)
   check_library_exists(c __cxa_thread_atexit_impl ""
     LIBCXXABI_HAS_CXA_THREAD_ATEXIT_IMPL)
-  set(LIBCXXABI_HAS_SYSTEM_LIB NO)
 else()
   check_library_exists(dl dladdr "" LIBCXXABI_HAS_DL_LIB)
   check_library_exists(pthread pthread_once "" LIBCXXABI_HAS_PTHREAD_LIB)
   check_library_exists(c __cxa_thread_atexit_impl ""
     LIBCXXABI_HAS_CXA_THREAD_ATEXIT_IMPL)
-  check_library_exists(System write "" LIBCXXABI_HAS_SYSTEM_LIB)
 endif()
diff --git a/libcxxabi/src/CMakeLists.txt b/libcxxabi/src/CMakeLists.txt
index 3decda3..06a9a5f 100644
--- a/libcxxabi/src/CMakeLists.txt
+++ b/libcxxabi/src/CMakeLists.txt
@@ -67,9 +67,7 @@ if (LIBCXXABI_ENABLE_FORGIVING_DYNAMIC_CAST)
   add_definitions(-D_LIBCXXABI_FORGIVING_DYNAMIC_CAST)
 endif()
 
-if (APPLE)
-  add_library_flags_if(LIBCXXABI_HAS_SYSTEM_LIB System)
-else()
+if (NOT APPLE) # On Apple platforms, we always use -nostdlib++ so we don't need to re-add other libraries
   if (LIBCXXABI_ENABLE_THREADS)
     add_library_flags_if(LIBCXXABI_HAS_PTHREAD_LIB pthread)
   endif()