[GlobalISel] Add the proper cmake plumbing.

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