[CMake] Mark runtime library link libraries as private

There's no need to expose these dependencies to consumers. This
matches the change made to other runtimes in D57456.

Differential Revision: https://reviews.llvm.org/D57873

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@353376 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/Modules/AddCompilerRT.cmake

@@ -280,7 +280,7 @@ function(add_compiler_rt_runtime name type)
         OUTPUT_NAME ${output_name_${libname}})
     set_target_properties(${libname} PROPERTIES FOLDER "Compiler-RT Runtime")
     if(LIB_LINK_LIBS)
-      target_link_libraries(${libname} ${LIB_LINK_LIBS})
+      target_link_libraries(${libname} PRIVATE ${LIB_LINK_LIBS})
     endif()
     if(${type} STREQUAL "SHARED")
       if(COMMAND llvm_setup_rpath)