Switch to LLVM 3.2 as the default for prebuilts

There are now prebuilt LLVM images for OS X 10.6, 10.7, 10.8 and Linux
versions based on GCC 4.1, 4.2, 4.3, 4.4, 4.5, 4.6 and 4.7, both for
i686 and x86_64 architectures. This should cover most of the systems
that people use.