Add a freestanding target

My experience with compiler-rt was a complete nightmare. So if it is possible to go without it - it's better to go without it.

Experience:
1. Code seems to be abandoned, for example, SjLj exceptions were added to libcxx but weren't backported to compiler-rt while they should be.
2. While working on proper backport I've "worked" with one of code owners. Worked in this case means I've pinged him every 2-3 days for about a month until full despair.
3. In one of enlightenment moments when we've been actually discussing my initial patch I was asked to complicate the code to look like libcxx counterpart, which might sound reasonable until you see [the actual code](https://github.com/llvm-mirror/libcxxabi/blob/master/src/cxa_personality.cpp#L618-L658) - try to see from the first look which variables are available when `__USING_SJLJ_EXCEPTIONS__` is defined and when not. And try to describe the program flow. And later on try to answer the question why it should be done in that way considering SjLj and "usual" exceptions share almost 0% common code in case of C exceptions.
4. This weekend I've spent trying to understand how is it possible that 3-month old changes to compiler-rt which broke all-but-iOS compilation (arm architecture), haven't been addressed at all.