Also link to -ldl to prevent undefined references.

--- a/libcxxabi/src/CMakeLists.txt
+++ b/libcxxabi/src/CMakeLists.txt
@@ -73,6 +73,7 @@
   endif()
 
   add_library_flags_if(LIBCXXABI_HAS_C_LIB c)
+  add_library_flags_if(LIBCXXABI_HAS_C_LIB dl)
 endif()
 
 if (LIBCXXABI_USE_LLVM_UNWINDER)
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -745,6 +745,10 @@
     if (LIBCXX_HAS_PTHREAD_LIB)
       target_compile_definitions(${target} PRIVATE -D_LIBCPP_LINK_PTHREAD_LIB)
     endif()
+    if (LIBCXX_HAS_C_LIB)
+      target_link_libraries(${target} PRIVATE dl)
+    endif()
+
     if (LIBCXX_HAS_RT_LIB)
       target_compile_definitions(${target} PRIVATE -D_LIBCPP_LINK_RT_LIB)
     endif()
