Push forward LLVM 3.4 support

This sequence of commits introduces both changes to support LLVM 3.4 as well architectural changes to the script to make it more flexible to use from the command line.

Specifically, it is now possible to build libcxxabi on 64-bit Linux, which [others had issues with in the past](http://clang-developers.42468.n3.nabble.com/Trying-to-build-libcxx-libcxxabi-on-Linux-64-bit-td4032792.html). Moreover, the select commits of the different projects make a stage 1 build possible. It took me a while to find compatible commits due complicated inter-project dependencies. However, the 3.4 build still fails at stage 2 now and I am trying to see if I can fix it. I am submitting this pull request nonetheless because it constitutes still a significant step forward in my eyes.