[CMake] Handle common options for runtimes build

All the existing runtimes relies on flags which are set by AddLLVM
and HandleLLVMOptions. In the standalone case, they would include
these themselves, but when being built using LLVM runtimes we should
include these in the top-level runtimes CMake files.

Differential Revision: https://reviews.llvm.org/D28389

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291590 91177308-0d34-0410-b5e6-96231b3b80d8