[CMake] Provide an option to disable runtimes build

This could be used to either disable the runtimes build altogether
or avoid building them but still generate the build targets.

Differential Revision: https://reviews.llvm.org/D31060

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298653 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed