enable cross compilation with cmake

This patch allows us to use cmake to specify a cross compiler: target different
than host. In particular, it moves LLVM_DEFAULT_TARGET_TRIPLE and TARGET_TRIPLE
variables from cmake/config-ix.cmake to the toplevel CMakeLists.txt to make them
available at configure time.

Here is the command line that I have used to test my patches to create a Hexagon
cross compiler hosted on x86:

$ cmake -G Ninja -D LLVM_TARGETS_TO_BUILD:STRING=Hexagon -D TARGET_TRIPLE:STRING=hexagon-unknown-linux-gnu -D LLVM_DEFAULT_TARGET_TRIPLE:STRING=hexagon-unknown-linux-gnu -D LLVM_TARGET_ARCH:STRING=hexagon-unknown-linux-gnu ..
$ ninja check

llvm-svn: 162219