[llvm] [runtimes][cmake] Keep default PIC behavior consistent in standalone build (PR #205797)

[Louis Dionne via llvm-commits]

https://github.com/ldionne requested changes to this pull request.

The original purpose of creating the runtimes build (`runtimes/CMakeLists.txt`) was to break free from the "common LLVM stuff", which didn't make sense for the runtimes. Then we started introducing variables to control the runtimes builds as `RUNTIMES_XYZ` when needed, and to use CMake builtin variables otherwise. In this case, it seems like we should be utilizing CMake's own `CMAKE_POSITION_INDEPENDENT_CODE` instead?

https://github.com/llvm/llvm-project/pull/205797