[CMake] Split the target side of runtimes build
Previously, llvm/runtimes/CMakeLists.txt played two different roles:
- host side which could used to set up the build of runtimes for different targets in the right order;
- target side to build the runtimes for the specified target.
This change splits llvm/runtimes/CMakeLists.txt and moves the target
side to runtimes/CMakeLists laying down the foundation for the "A vision
for building the runtimes" proposal. From the user perspective, there
shouldn't be any visible difference at the moment.
Differential Revision: https://reviews.llvm.org/D93408