High memory usage during compilation when using built-from-source LLVM 6.0

[sppalkia]

As mentioned in #476, using LLVM 6.0 when built from source seems to increase rustc compilation times dramatically and also seems to increase memory usage significantly (the linker and dsysutil use > 2GB of memory per integration test module, for example). This is most likely because of the flags with which LLVM is compiled.

This issue doesn't seem to occur on a "standard" distribution of LLVM (via apt-get on Linux or brew on MacOS).