Initial, very rough cut at a new CMake build system for compiler-rt.

Initial, very rough cut at a new CMake build system for compiler-rt.
Some high-level notes:

1) An explicit goal is to support building compiler-rt as a subproject
   build, checked out into the projects/compiler-rt directory. There are
   many other possible ways of building the code here, but this is
   optimized for development on LLVM/Clang/compiler-rt, and incremental
   trial and testing of the toolchain.
2) The current support is targeted at Linux. I would love to see this
   generalized to other platforms, but for the sake of simplicity in
   testing, I'm focusing here first.

Much of this patch was paired with Manuel, and I credit him with the
majority of the work here.

Some important caveats that I'll be working on in subsequent patches:

1) This uses the host compiler rather than using the just-built-clang.
2) Currently only x86 is supported.
3) Currently, none of the tests are built or run.
4) Uses CMake's builtin globbing which doesn't update correctly.
5) This is still turned off from LLVM's CMake build until these issues
   are addressed

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@154060 91177308-0d34-0410-b5e6-96231b3b80d8