[libc++] Use PRIVATE to link benchmark dependencies

It's better style to use PRIVATE when linking libraries to executables,
and it doesn't make a difference since executables don't need to propagate
their link-time dependencies anyway.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374050 91177308-0d34-0410-b5e6-96231b3b80d8