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.
rustc: Hint to the linker about static/shared libs

If a linker finds both a static and a dynamic version of the same library, then
the linker often chooses the dynamic version. This is surprising when a native
library is specified as being "static" in rust source. This modifies the linker
command line to obey the hints given in rust source files and instructing the
linker to prefer a particular version of a found library.

Unfortunately, this patch has no effect on osx because the linker supports
no such hint, and it also has no effect on windows because the linker apparently
just ignores it. For now this is predominately used to enable the previous patch
of linking to libstdc++ statically, but more support would need to be added for
this in the future if we wanted to officially support it.

cc #12557 (doesn't close because it doesn't support OSX and windows)