[CMake] Split the target side of runtimes build

Previously, llvm/runtimes/CMakeLists.txt played two different roles:
1. host side which could used  to set up the build of runtimes for
   different targets in the right order;
2. 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

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