llvm: Add an option to statically link libstdc++

The goal of the snapshot bots is to produce binaries which can run in as many
locations as possible. Currently we build on Centos 6 for this reason, but with
LLVM's update to C++11, this reduces the number of platforms that we could
possibly run on.

This adds a --enable-llvm-static-stdcpp option to the ./configure script for
Rust which will enable building a librustc with a static dependence on
libstdc++. This normally isn't necessary, but this option can be used on the
snapshot builders in order to continue to make binaries which should be able to
run in as many locations as possible.