This patch adds the necessary plumbing to cmake to build the sources related to
GlobalISel.

To build the sources related to GlobalISel, we need to add -DBUILD_GLOBAL_ISEL=ON.
By default, this is OFF, thus GlobalISel sources will not impact people that do
not explicitly opt-in.

Differential Revision: http://reviews.llvm.org/D15983

llvm-svn: 258344