Introduce CMake - a Cross-Platform Makefile Generator

Why? Although Makefiles are cool, CMake brings some nice features:

 * cross platform: possibly build baselibc on Windows with IAR and
   mingw32-make (when IAR support is merged in CMake)
 * cross-compilation: use a CMAKE_TOOLCHAIN_FILE rather than
   hardcoding toolchain info into the Makefile.
 * easier "embedding" into super-projects:
   add_subdirectory(baselibc)

So, this patch adds a CMakeLists.txt file that builds baselibc and
unittests.

Example usage:
  mkdir build
  cd build
  cmake ..
  make  # builds libc.a and the 'tests' executable

Unittests are integrated with CTest:
  make test  # runs the registered tests (just the 'tests' executable for now)