[CMake] Produce an empty library for GlobalISel when not building it.

The rational for this change is that LLVMBuild cannot express conditional 
dependencies. Therefore, when we start optionally using GlobalISel library for 
say AArch64, without that change, all the tools that use the AArch64 library 
would need to explicitly link with GlobalISel when we ask for it.

This does not scale.

Instead, we will set the dependencies between the target and GlobalISel and if 
we did not ask to build GlobalISel, the library will just be empty.

Thanks to Chris Bieneman and Mehdi Animi for the idea.

llvm-svn: 260566