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). While the script runs to completion for all stages, the `testit` script still fails:

    /usr/bin/ld: /tmp/test-4e6344.o: undefined reference to symbol '_ZNSt12length_errorD1Ev'
    /usr/bin/ld: note: '_ZNSt12length_errorD1Ev' is defined in DSO /home/matthias/opt/llvm/bin/../lib/libc++abi.so.1 so try adding it to the linker command line
    .../opt/llvm/bin/../lib/libc++abi.so.1: could not read symbols: Invalid operation
    clang-3.5: error: linker command failed with exit code 1 (use -v to see invocation)

It seems that using libcxxabi added the requirement of having `-lc++abi` as part of the compile command. Do you see a way around this?