[libc++] Make sure we link all system libraries into the benchmarks

[libc++] Make sure we link all system libraries into the benchmarks

It turns out that r374056 broke _some_ build bots again, specifically
the ones using sanitizers. Instead of trying to link the right system
libraries to the benchmarks bit-by-bit, let's just link exactly the
system libraries that libc++ itself needs.

llvm-svn: 374079