[libc++] Re-export libc++abi symbols on Apple platforms when using system-libcxxabi (llvm#77218)

When using LIBCXX_CXX_ABI=system-libcxxabi on Apple platforms, we would not
re-export the libc++abi symbols unlike when LIBCXX_CXX_ABI=libcxxabi. This
was caused by overly strict string matching in CMake.

NixOS/nixpkgs#269548

Author: a-n-n-a-l-e-e

libcxx/src/CMakeLists.txt

@@ -230,7 +230,7 @@ if (LIBCXX_ENABLE_SHARED)
   # Maybe re-export symbols from libc++abi
   # In particular, we don't re-export the symbols if libc++abi is merged statically
   # into libc++ because in that case there's no dylib to re-export from.
-  if (APPLE AND LIBCXX_CXX_ABI STREQUAL "libcxxabi"
+  if (APPLE AND LIBCXX_CXX_ABI MATCHES "libcxxabi$"
             AND NOT DEFINED LIBCXX_OSX_REEXPORT_LIBCXXABI_SYMBOLS
             AND NOT LIBCXX_STATICALLY_LINK_ABI_IN_SHARED_LIBRARY)
     set(LIBCXX_OSX_REEXPORT_LIBCXXABI_SYMBOLS ON)