Another big step toward a viable CMake build system for CompilerRT,

ASan, and friends.

This explicitly switches the CompilerRT CMake build to require CMake
version 2.8.8 or newer which provides first-class support for "object"
libraries which consist of a pile of '.o' files -- exactly what is
desired for composing runtime libraries. I've gone ahead and switched to
using this.

I've also added the interception library which I missed initially. And
I've added proper dependencies between the various libraries. With this,
I'm able to build archives for asan that appear to contain all of the
necessary .o files.

The final tweak here is to start setting up the compile flags and macro
defines expected by ASan and its helper libraries. These may not be
entirely correct currently, they're based loosely on my reading of the
old Makefiles. However, they can be tweaked more easily now that they're
wired up properly.

llvm-svn: 159129