Fix warning about mismatched target tuples in KLEE

[alastairreid]

This is thought to be benign but it would be nice to fix it.

The problem is that KLEE links some libraries with the bitcode it is verifying.
Those libraries are currently compiled with the standard version of clang.
To fix the mismatch, we need KLEE to use a version of clang built using the same LLVM that Rustc uses.

Rustc does not need clang so, even though it includes clang as a submodule, it does not build it.
It is not clear (I tried!) how to get the rustc build process to build a copy of clang.